I came across the same in R2020b and found the reason; for what it's worth, and in case it helps someone else, below are my notes:
Regarding the odd initialisation with a call to a set function - that is not even defined in the custom code header files - I found it to be through this initialisation value (in my case in the DataStoreMemory block, but perhaps somewhere similar if implementing differently);
Other data stores use a nominal integer (of 0) with a datatype convert out of the “read” block, and the Simulink.signal link has a uint8 base datatype (before the convert to enum) - corrected and codegen’d successfully.
This has removed it from the init function and into the top of the Rx function, where the get_ActSystype() is called in cgASW_204_Lecu.c line 2179.
Not sure this is the best example implementation of GetSet, but it does at least work.
