Dear Chippy Augustine many thanks for your interesting technical question. In this context please see the answers to the following closely related RG threads entitled

1. What are the scientific reasons behind the increase in mass during the TGA of a compound?

https://www.researchgate.net/post/What_are_the_scientific_reasons_behind_the_increase_in_mass_during_the_TGA_of_a_compound

2. Why my TGA graph shows a weight gain?

https://www.researchgate.net/post/Why-my-TGA-graph-shows-a-weight-gain

3. TGA graph shows a weight gain even run a blank sample. Why?

https://www.researchgate.net/post/TGA_graph_shows_a_weight_gain_even_run_a_blank_sample_Why

I'm confident that you will find suitable answers which will help answering your question.

For more general information, please also have a look at the following "Beginner's Guide to Thermogravimetric analysis (TGA)":

https://www.perkinelmer.com/lab-solutions/resources/docs/GDE_TGABeginnersGuide.pdf

Dear Chippy,

What was the inert atmosphere? Do you mean under N2 or sth? Depending on size of the voids, it is possible for MOF to absorb the inert gas. Especially in the beginning of TGA, when the structure has not been decomposed yet.