As simple as that, when you rigth click multiple times and fast the pouch the coins inside duplicate untill the number gets to 8 or the max stack size.
Im playing on a server using Treasure2-mc1.16.5-f36.2.0-v1.7.3

Thanks if someone reads this and can fix or help me to fix it server side if there is a way.

Is fixed in v2.2.0 (will be released shortly)