Attempting to assign one enum reg to another of the same type gives the following error:
fsm_spi_control.sv:35: error: Enumeration type mismatch in assignment.
1 error(s) during elaboration.
You seem to have CSS turned off.
Please don't fill out this field.
This does look like a problem in the blocking assignment code, but you really should be using a non-blocking assignment (<=) at that location and a non-blocking assignment does appear to work as expected.
I have pushed a patch that fixes this problem in development.