Alt-tabbing the game in windowed mode can sometimes cause framerates to be locked at a lower value (60fps from feel) on a 165Hz monitor. So far this framerate issue is only fixed on restarting the PC. Restarting Minecraft or reloading the world does not fix this issue and persists between play-sessions.

Repro Steps

1. Have Minecraft in Windowed mode, running on a high (atleast >60Hz) refresh-rate monitor (The higher the more noticable this bug is).
2. Load into a world.
3. Alt-tab away and back to the game repeatedly until issue occurs.