Mixnodes only forwards compatible, not backwards.

[Maneki]

When I create a scene in 3.3, save it and load it in 3.4, the mix nodes keep working. When I save it in 3.4 and go back to 3.3 the node is “undefined”.

‌

[Alessandro Padovani]

Unfortunately this is normal and it’s how at blender they want it. Not a bug.

[Thomas Larsson]

I’m surprised that you could even open the file in blender 3.3. In my experience trying to open a file in an earlier version usually results in an error.

[Maneki]

Blender functionality.