There are at least two pull requests I could find trying to address this.
https://github.com/EpicGames/UnrealEngine/pull/7825
https://github.com/EpicGames/UnrealEngine/pull/10048
My take on why neither was merged is that the function is intended to be used with floating points (where Min != Max) rather than integral types. So rather than changing the existing function that may very well work as intended, it would probably be better to add another one for integral types and choose the appropriate one via enable_if or similar pattern.
With floating point the + 1 suddenly doesn’t make sense anymore. Taking the OP’s example:
Considering Min = 0.0f, Max = 5.0f :
for X = 5.0f it returns 5.0f
for X = 6.0f it returns 1.0f
for X = 5.1f it returns 0.1f (would be -0.9f with + 1 to size)
for X = 6.1f it returns 1.1f (would be 0.1f with + 1 to size which is clearly wrong)