Always include

• A log if the issue is casing a crash. These are found in your instance directory under /logs
  https://gist.github.com/AzureDoom/5e638f968c1e16155636fac015ac1fcc

• Mod version, MC version and Loader you are using
  2.2.8/1.19.2/Fabric

• A description of your issue and how to reproduce

• Throw Flex Seed into lava, server dies.

Flax seeds are just a normal item, they should not perform any such thing, especially calling a method from their block. Do you have any mod that plants stuff from seeds entities? Fabric doesn't even show mixins so I can't tell

Maybe same as this one? https://www.curseforge.com/minecraft/mc-mods/supplementaries/issues/449

Just had to troubleshoot the same issue, in our case it was the selfPlanting option of the vanillatweaks mod

A bit late but should be reported to them