The quality and completeness of an automated identification of citations by algorithms depend mainly on the quality of these algorithms (an their programming) and on the used dataset of full texts of publications. Different platforms use different algorithms and different datasets of publications, and none of them is perfect or complete. Therefore, it is practically impossible to find all citations. Citations my "disappear" when, e.g., duplicate entries for one and the same publication are eliminated from the dataset. The cause for missing citations is one of the most frequently asked question at ResearchGate. See these discussions (just a selection):

ResearchGate:

https://www.researchgate.net/post/RG-citations-how-are-they-calculated/1

https://www.researchgate.net/post/missing_citation_number/1

https://www.researchgate.net/post/Why_are_some_of_my_citations_not_shown2/1

https://www.researchgate.net/post/Why_the_citation_is_not_shown/1

https://www.researchgate.net/post/How_can_we_add_citation_on_research_gate/1

https://www.researchgate.net/post/How_add_references/1

https://www.researchgate.net/post/Citations_from_RG/1

https://www.researchgate.net/post/citations_in_RG/1

https://www.researchgate.net/post/How_do_you_fix_missing_citations/1

https://www.researchgate.net/post/How_to_add_missing_citation/1

https://www.researchgate.net/post/What_if_I_have_citations_in_the_research_gate_indexed_papers_that_research_gate_does_not_show_them/1

https://www.researchgate.net/post/Question_about_article_submitted_and_references_does_not_appear/1

https://www.researchgate.net/post/RG_system_to_detected_Citations/1

ResearchGate and Google Scholar:

https://www.researchgate.net/post/In_Google_Scholar_my_master_thesis_got_a_citing_but_in_Researchgate_that_does_not_visible_Why_does_it_happen/1

https://www.researchgate.net/post/Can_someone_help_me_understand_why_there_a_difference_in_the_number_of_citations_shown_in_a_persons_Researchgate_and_Google_Scholar_account/1

https://www.researchgate.net/post/Why_is_citation_count_of_research_articles_different_in_Research_gate_from_that_of_google_scholar/1

https://www.researchgate.net/post/Why-does-my-article-citations-appear-in-google-scholar-but-not-included-in-research-gate/1

https://www.researchgate.net/post/Why-some-citations-are-not-recorded-which-are-found-in-google-scholar/1

https://www.researchgate.net/post/Which_is_more_accurate_in_reflecting_research_outputs_Google_Scholar_or_ResearchGate2/1

https://www.researchgate.net/post/Does_anybody_know_how_we_can_add_the_missing_citations_in_Research_Gate_as_will_as_Google_Scholar_profile/1

https://www.researchgate.net/post/Does_anyone_know_how_long_it_takes_for_Google_Scholar_to_pick_up_new_citations_from_ResearchGate/1

https://www.researchgate.net/post/Recent_publications_and_Citations_on_ResearchGate_are_not_getting_indexed_in_Google_Scholar/1

https://www.researchgate.net/post/Is_there_any_method_to_add_missing_citation_of_your_work_in_RG_As_most_of_the_papers_have_more_citations_as_per_google_scholer_but_very_low_in_RG/1

https://www.researchgate.net/post/How_to_include_all_citations_on_RG_and_Google_scholar_citing_your_manuscript/1

https://www.researchgate.net/post/How_can_we_add_citations_in_google_scholar_and_Research_Gate_for_our_articles/1

https://www.researchgate.net/post/How_can_I_link_my_articles_on_research_gate_to_google_scholar/1

https://www.researchgate.net/post/How_to_link_Research_Gate_article_citations_to_Google_Scholar_and_vice-versa/1

https://www.researchgate.net/post/I_have_4_citations_in_Google_scholar_can_these_citations_be_captured_by_RG/1

https://www.researchgate.net/post/How_to_add_missing_citations_on_Google_scholar_Research_gate_and_Scopus/1

https://www.researchgate.net/post/Does_any_know_how_can_I_add_missing_citations_to_google_scholar_and_resaerch_gate/1

Google Scholar:

https://www.researchgate.net/post/Does_anybody_know_how_we_can_add_the_missing_citations_to_our_profile_in_Google_Scholar/1

https://www.researchgate.net/post/Is_there_any_to_add_the_missing_citations_in_Google_Scholar_Profile/1

https://www.researchgate.net/post/How-to-add-missing-citations-on-Google-Scholar/1

https://www.researchgate.net/post/How_can_i_add_missing_citation_to_my_Google_Scholar_profile/1

https://www.researchgate.net/post/The_article_published_in_a_scientific_journal_within_the_Scopes_container_Why_didnt_a_new_citation_appear_in_Google_Scholar/1

https://www.researchgate.net/post/Could_you_please_explain_my_inquiry_on_citations/1

https://www.researchgate.net/post/What_to_do_if_a_citation_is_not_included_in_Google_Scholar/1

https://www.researchgate.net/post/Why_some_article_citations_is_not_seen_in_the_google_scholar/1

https://www.researchgate.net/post/Do_you_know_why_some_citations_are_not_index_in_the_google_scholar_profile_but_the_same_citation_index_in_the_other_data_bases_such_as_scopus/1

Scopus:

https://www.researchgate.net/post/Does_anybody_know_how_we_can_add_the_missing_citations_to_our_profile_in_Scopus/1

https://www.researchgate.net/post/How_to_add_missing_citation_and_missing_papers_at_Scopus/1

Thanks for your reply.

"Citations my "disappear" when, e.g., duplicate entries for one and the same publication are eliminated from the dataset".

Duplication will happen when one has listed same paper twice. In my google list of paper, there are no duplication. At the same, the decrease CAN'T be of 70 citations.