Hmm, one odd thing I may suggest to try is to clear your shader cache and let it recompile. Could be something weird with a patch. Otherwise, I am not sure
edit: found a different thread saying to untick and retick pip border but the bug reproduces itself every game you join
I have customized the crosshair to my liking, and i have noticed that it changes by itself when i join new game.
The only way to revert it back to it's original style is to mark and unmark the show pip border option in crosshair options.
I will also add photos to make it easier for you to visualise this issue.
(The white one is how it's suppoused to look like)
(The black one is what it changes to when i join a new game)
View attachment 34896