Fix: I was having this issue with my laptop when it is using its internal display, it seems like it takes the vsync setting from the intel integrated graphics but runs the game on my gtx 1060. Sadly I cannot find a way to disable vsync on intel drivers as the settings seem to be non existent for UHD 630 graphics. When hooked up to an external display it is directly linked to my 1060 which has vsync off, so it went back up to 300fps
It seems like an intel issue with having terrible graphic settings but it would be nice if minecraft just had these settings like java edition.
MCPE-163023 has the exact same issue. The issue needs to be resolved.
I can't recrete the issue, it just seems to happen randomly when you sometimes load up the game for the first time.