Moving a script file to a custom assembly with a .asmdef file will break serialized instances of classes that are inheriting from a base class.
If the base class is abstract, the only available instance to choose from when creating a new instance via Odin is Null, otherwise it is only the base class (even if there are multiple inheriting classes that would be choosable.
Here are a series of screenshots demonstrating the issue:
Project Layout -> https://i.imgur.com/4Eaaw6l.png
Simple inheriting classes -> https://i.imgur.com/T0RDWQd.png
With the .asmdef, only Null can be selected -> https://i.imgur.com/uoyb8dL.png
Moving the .asmdef -> https://i.imgur.com/TXJ55gZ.png
Sub-classes can be created -> https://i.imgur.com/L3DgJ5t.png
Editor only mode is disabled.
Windows 10 64 bit
Odin Inspector v.2.0.6