LixaSL hey there! I was searching trough internet and found some guide
If you have a texture size 51x9 then correct format should be 64x16 (64x8 but mip maps will be decreased from 4 to 3)
After my research even mods cannot fix it and user have to fix it.
EDIT: It’s java behaviour and would be great to see an updated version of plugin
Please update to 3.6.4r3