If you enable Vsync then restart the game,your frame rate will not be synchronized to refresh rate unless you reset Vsync option again.

Same Issue here ! Vsync seems to be inactive until you turn it off from on again and then turn it on again. and that every time you restart the game and join a world.

after some testing i found out thats a bug with Forge itself.

i sent this bug to the developer of the Embeddium mod (Fork of Rubidium) and he made a workaround. it will be included in the next update of the mod, if you want to use it now just go to the bug report of me at Embeddium there is a link of the new version with workaround.