Ah, thank you for pointing that out. I can't believe I didn't see it!
Can confirm in 1.20.5 (experimental features enabled).
To clarify, the issue is that right clicking the armadillo with the brush causes the lead to come off instead of a scute to drop. Nevertheless, the advancement is granted.
It's possible that the real bug is that the lead breaks in the first place (I initially expected the brush to collect a scute), but this behavior is consistent with other mobs (e.g., attempting to shear a sheep or milk a cow while the animal is on a lead removes the lead instead).
This "bug" seems to be a misunderstanding of breeding mechanics and should be closed
— ZEnderMan333 added a comment - 27/Mar/20 2:00 AM
— SO! I FOUND THE PROBLEM! THIS IS NOT A BUG!
— In order for villager breeding to occur, the villagers breeding have to be able to access the 3 beds, or at least be able to sleep in them. If the beds are on a ledge above the villager, THE BREEDING WILL NOT HAPPEN.
— SO YES, WE CAN THANK MOJANG FOR MAKING VILLAGER BREEDING 100X HARDER.
I do not experience this bug playing 1.20.4 or 1.20.5. This issue may have been resolved without updating the bug report.
According to the wiki (https://minecraft.wiki/w/Armadillo#Drops), the default drop rate is one scute every 5-10 minutes (the same rate as chickens laying eggs). In an hour and a half (90 minutes), you should have collected between 9-18 scutes per armadillo (somewhere between 6 and 11 armadillos will give you a stack and a half in this time). If you collected significantly more scutes or had fewer armadillos, this is likely a bug.
Can confirm in 1.20.5. Also duplicated by MC-271211.