Minecraft v1.16.5
Forge v36.1.32
Quark r2.4-315
Mouse Tweaks v2.14

When the Mouse Tweaks mod is installed, the right-click function for the seed pouch & shulker boxes doesn't work and items cannot be inserted, but only extracted. After removing Mouse Tweaks, I was able to use the seed pouch normally.

This is known. You can sort it out in your end by disabling RMBTweak in their config.