Unreal Engine 4 apparently didn't cause problems for PUBG players only as players have now discovered that similar issues are happening in Fortnite when it's played at 30 FPS - some guns with high rate of fire start shooting slower.
PUBG has recently seen some that go against the "esports ready" mantra. Namely, players with lower frames per second have lower recoil on the fast firing weapons in the game, but it was later discovered that the guns actually shoot slower, which is the reason of lower recoil.
Now it appears that the same issue has been present in Fortnite all along as well. One of the high-profile streamers who played both games, DrLupo, decided to test how Fortnite handles similar tests. His tests were a bit simpler than those performed by WackyJacky101 for PUBG, but they served the purpose.
You can clearly see in DrLupo's that the same SMG shoots faster on the uncapped framerate than it does on 30 FPS. If it's not clear initially, there is a side-by-side comparison at the end of the video in the aforementioned tweet, which clearly shows four to five bullets still in the magazine on the 30 FPS side at the moment of 300 FPS gun running dry.
Epic Games quickly to the tweet, stating that they are aware of the issue and are "investigating a fix for it". Depending on how complex the issue is with coding, we could be looking at anywhere from mere days to several months until it is resolved.
On the bright side, Fortnite is much better optimised than PUBG, so it is more likely to perform at 60FPS which shouldn't cause any issues for the players.
Console players may find themselves at a disadvantage with the problem, but they can always choose to opt out of crossplay, at least until the FPS-ROF relation issue gets fixed. It may not be optimal, but the workaround will at least help players compete on a level playing field.