THIS IS AN UPDATED POST:
It seems that there is a missing Term Mapping option in Form Settings / Data / Populate Using Action, like there is in Actions / Post Management options panel.
Surprisingly, there is a Tag Mapping option in Form Settings / Data / Populate Using Action options panel, which is absent in Actions / Post Management options panel.
So, there is an obvious mistake in the code design.
Due to this, when I try to use WS Forms to edit an earlier published post, using your Populate a Form With Post Data instructions, I cannot retrieve the custom category of a custom post type. This makes this particular function unusable 🙁
Could you please look into this?