[sjrd]

Only comments on the commits are lost. Comments on the Files tab of the PR itself are kept and you can "View outdated diff".

In our project we exclusively use comments on the Files tab precisely for that reason.

[EdiX]

Well, I guess this explains why sometimes I saw comments on PRs disappear and sometimes they didn't, thanks.

The difference between commenting on the commit page and on the "files tab" is pretty subtle though...