I can confirm this unless there's some voodoo way to do it. I tried the add the flashlight to the M4 and FNX45.

No, this seems to be working as intended. If you have the correct hand guard for the m4 it attaches no problem. There is one that looks like a picatinny rail system. I do not remember what it is called, nor can i log into a server atm to check however. I can confirm with this hand guard it will attach.

Yes, with the right handguard it will attach!

I attached the *MP* (magpul) handguard to my M4, and the weapon light would not attach. Are there picatinny rails you can add to it for direct attachment (since this handguard just has the holes, but no rails)? Perhaps finding and adding rails turns the MP into the RIS handguard??

*edit* it attaches to the RIS just fine for me, which makes me wonder what the purpose of the MP handguard is in the mod if there aren't separate rails you can attach

not a bug, you can attach flashlights to weapons with compatible rail systems.

Hello tamaster92 and thank you for the report.
To attach flashlights to weapons the players require specific rail attachments.
Regards,
Geez