Ideas
The cause seems to be the underbarrel.
It seems that customizations are reverted on weapons such as the L110, DRS-IAR, and M250, which have a bipod attached as an underbarrel by default.
#Steps to Reproduce
1. Remove the bipod (underbarrel).
2. Equip attachments until your score reaches 100.
3. End a match using any rule.
4. The reversion occurs when you return to the lobby or are automatically rematched.
This is just a guess, but I think the internal processing works something like this: "For weapons that have an underbarrel attached by default, if a match ends with nothing attached to the underbarrel, re-equip the underbarrel attachment. If your score exceeds 100 at this point, remove the last attachment attached."
The workaround for this issue is to keep your score under 100 with an attachment attached to the underbarrel.
I'm also having trouble with this issue, so I hope it gets fixed.