I still have no idea what the problem is, the expected behaviour is to mark a region, and have the marked region surrounded in whatever markup is needed. Unless I'm blind, this is just what is happening.
It's entirely possible that something in my configuration is interacting poorly with the mode, if this is the case it will probably be safe to ignore it. (I would try with ‘emacs -q’ but I use EXWM with a login manager which makes that pretty awkward) Here are steps to reproduce, type the following string and then enter the following command sequence:
The quick brown fox jumps over the lazy dog.
C-a C-SPC C-u 10 C-f C-c C-f C-s If these are bugs then the output should be as follows with brackets indicating the cursor location:
~~~~The quick br[o]wn fox jumps over the lazy dog.