Minecraft Version

1.20.x

Version

1.106.1

Details

As of version 1.106.1 farming turtles can not till dirt below them. This effects fabric and forge version.
To replicate:
-Install 1.106.1
-Place farming turtle 1 block above dirt
-Try to till land below
-Observe it can still till dirt in front of it

Version 1.105 is unaffected

I seem to remember that turtles will only till dirt if the resulting farmland would be able to become hydrated. Have you tried with a water source nearby?

There were a couple of issues introduced with tools in the latest version. These are now fixed (see #1527), but not yet released.