There are two potential catalysts behind this issue, with my money on it being a combination of the two.
First off, the accounts you are buying are likely of a dubious quality and hence get check-pointed quite easily, especially when in conjunction with point #2. This is common among accounts registered via automation which make up roughly 85% of the entire accs market, even among those who claims theirs were hand-made.
Point #2: Certain account changes elicit a greater amount of vigilance from the algorithm. In my experience, the hierarchy goes something like this:
1. Password change
2. Password/username change
3. Name change
4. Bio/link change
5. Profile pic/post uploads
The higher up the hierarchy the change is, the greater of an effect it will subsequently have on an account's trust score.
My suggestion would be to stop changing usernames and see what the result of that will be. Your accounts can still be of a very good quality by automation standards but that change can incapacitate even the best of them so I try steer clear of it when rebranding accounts.