The FPS in the F3 screen will not always reach the value set for the maximum FPS under video settings. An example: if the maximum FPS is set to 120, the FPS shown in the F3 screen will only go up to 116, whereas in 1.12.2 it would just go up to 120 just fine. If the maximum FPS is set to 10, it will reach 10 FPS just fine in the F3 screen.

I tested this with both vsync off and on, this does not make any difference.

Here are some more max FPS settings I tested:

Max FPS setting                     FPS in F3 screen

30                                           30

40                                           40

50                                           49

60                                           59

70                                           68

80                                           78

100                                         96

120                                         116                                         

However, once I change the max FPS setting and I click back to game, the FPS in the F3 screen actually goes up to the max FPS value for just a second or two.

I already reported this bug earlier (MC-130881) but it got resolved for an invalid reason in my opinion. It got resolved because the mod thought that the bug was about the difference between 60 and 59,997 fps. It is not, because the bug occurs on other max FPS values too.