I don't think there are many "AAA" game development studios in Poland, and CD Projekt RED is probably the most well-known internationally. I would hardly say that being a tester on Cyberpunk would be damaging to one's career, as other studios are going to be aware that the game's problems are largely down to management rushing out a release well before the game was ready.
That's likely mainly due to the nature of their development cycle. So far, CD Project has mainly just developed one big game at a time, with minimal overlap in-between. So unlike some other big studios that continuously have multiple games in various stages of development, where they can immediately shuffle employees to another title once their work on one is completed, they instead have periods of time when many of these employees are simply not needed. It's probably not so much that they "leave" as that they were only hired on as temporary help to begin with. What's a beta tester supposed to do when the game they are testing is finished and the next one is mostly just a collection of art assets and storyboards? While the gap can be somewhat filled with work on DLC and small spinoff titles, those generally won't require as large of a team as the initial release. Of course, for this game, they might be keeping more of the crew around for a while, as the game isn't actually "finished" yet.