It is pretty strange how it is the area facing the chamber that got chipped off.
I think normally you will see the side facing the trigger gets the most wear and tear as it is the side that holds back the BCG.
Maybe it was the magazine issue?
Maybe over insertion of the magazine?
Maybe there was something in between the magazine body and the bolt catch lug when you inserted the magazine, which can break the lug off.
I don't see how you can break the bolt catch any other way...
One of my SR15 Mod1 lower now has slightly over 30,000rds through it without any problem and has no broken parts.
(Buffer spring was changed to Springco white spring early on)
Is this Mod0? Mod1? Mod2?
How many rounds through the lower?