Glad to hear that you have it working.
In pjsip, when you have Registration Send, the default equivalent values are:
Username:Secret@SIP Server/Contact User
If Contact User is left blank, it defaults to Username.
There are several Advanced settings to modify these, but if you left those blank, your registration should be identical to what you have with chan_sip. You can compare the REGISTER requests to see if the same info actually gets sent.
If you don’t mind, could you please post a log (with the same info as you posted previously) for a chan_sip incoming call? Possibly, there is a bug in pjsip that is worth reporting, or there is a required pjsip setting that will be obvious from the comparison.
To enable SIP logging with chan_sip, use
sip set debug on