Chris:
We are seeing the same issue in our Portals here.
We are pretty sure it is related to using AD/IWA (Integrated Windows Authentication.)
In fact, given the work around, I don't see what else it could be.
We have a DEV/Test/Prod setup, all three are all in one servers (Portal, Data Store and ArcGIS Server are all running on same server, which are VMware (Vsphere) virtual servers (Win2K 2012R2.) All three are using the web adaptor under IIS.
Test and Prod both are set to use IWA and both fail when publishing a Survey123 in the same manner you describe. The Feature Service gets published but the Form does not.
The work around we have found is to:
Turn off IWA, publish your Survey123, Turn IWA back on.
1. Using IIS Manager on your Portal server, Navigate to your Portal website and open Authentication.
a. Disable Windows Authentication
b. Enable Anonymous Authentication
2. Publish your Survey123 using a publisher account on Portal that is not your AD login.
3. Go back to IIS Manager and reverse what you did in #1 to turn IWA back on (Enable Windows Authentication and Disable Anonymous Authentication.)
You should end up with your Survey123 published (both Feature Layer and the Form) and IWA working.
Obviously, probably need to do this after hours when others are not on your Portal.
Not an optimal fix for sure but it does seem to work for us.
Hope this works for you.