I think it might be due to excessive ProtK treatment. I would test different time/concentrations. For my experiments, I only had to use ProtK after fixation and then postfix again. Generally don't use ProtK for embryos younger than 24hpf.

For early embryonic stages, I sometimes fix prior to dechlorination and then dechorionate manually with forceps, which helps to preserve embryos.

Hope it helps.

Thank you for your answer, Anastasia,

Actually, I used our protocol previously, and was working well, and suddenly I see this effect.

I will try to adjust the Pro K treatment.