Wow that's fun, and yeah I can reproduce it here: https://chromium-review.googlesource.com/c/553397/

I have no idea why this works for most usernames but not rsesek.

While I try to track this down, you can use the full email address "rsesek@chromium.org" as a workaround, and that should work.

The response that gerrit gives when trying to add "rsesek" as a reviewer is
rsesek is not a valid user identifier
which isn't surprising -- what's more surprising is that "sunnyps" *is* a valid user identifier. It's clearly doing some sort of disambiguation when there's only one account with a given username portion or something like that.

I would guess that this is because rsesek has two accounts (one chromium, the other google), and so it doesn't know which to pick. Except that it successfully added alokp and boliu in your second CL, despite them both having two accounts as well.

I'm trying to follow up internally here: b/63115075

The internal bug has had a fix landed. I'm going to mark this fixed, though the actual fix likely won't be deployed until early next week.