I get what you're saying, and it doesn't really make sense why it
wouldn't work. Like you've already mentioned you are using Shelter
without Play Services, and I'm sure there have been multiple more
users doing the same.

Maybe it's something with OxygenOS 13 maybe that needs Play Services
for Shelter specifically? It is very arbitary and doesn't make sense,
but that's the only way to reproduce it that I've found so far. It is
safe to say it is unexplainable for now (maybe something could be
found in the logcat later?).

Side note, I also tried the
https://play.google.com/store/apps/details?id=com.afwsamples.testdpc
app mentioned on the Insular wiki here:
https://secure-system.gitlab.io/Insular/faq.html . The app is able to
open the work profile setup screen as well.

Regards,
S