Testet using only Accessories 1.5.3 and Library 7.0.8 on Forge 1.20.1.
The Certificate of Taming appears to increases the stats of the tamed entity, but after Exiting and Re-Joining the world, the stats revert back to normal.
This happens both on a Server and in Singleplayer.
I have tested it with taming Wolf and Horses.
Testet using only Accessories 1.5.3 and Library 7.0.8 on Forge 1.20.1. The Certificate of Taming appears to increases the stats of the tamed entity, but after Exiting and Re-Joining the world, the stats revert back to normal. This happens both on a Server and in Singleplayer. I have tested it with taming Wolf and Horses.