If bug:

• Forge Version: 1.15.2 - 31.2.31
• Mod Version: FruitTrees-1.15.2-1.4.1
• Can you reproduce this issue with relevant mods only?: yes

Both MixinBootstrap and Kiwi are installed, but the hybridization module doesn't seem to be active in any way.

Can you see the JEI category?

No, and the crafting table recipes for the saplings are present

Check config/kiwi.toml and see if the hybridization module is disabled

I reinstalled the 3 mods to test this and it worked this time... Not sure what I was doing wrong the first time, sorry for the false report.