Intune - Automatic enrolment by same user removes previous device enrolled by same user

I'm testing automatic enrolment to Intune with hybrid-joined VMs and am running into an issue that I can't figure out.

What I'm experiencing is that I can automatically enrol client A using the GPO. It successfully hybrid-joins, gets synced up to AzureAD and becomes managed by Intune. However, if I enrol client B with the same user I see that client A gets unmanaged from Intune but its device account remains in AzureAD as hybrid joined.

It's acting like a single user can only enrol a single device and any subsequent device takes the previous license away. I'm using the EMS E5 trial although I don't think that's relevant.

My understanding is that by default I should be able to enrol up to 15 devices per user but device enrolment limits don't apply to GPO-based automatic enrolment if I've read the documentation correctly. So I have no idea what's going on.

Any suggestions as to what I'm missing here?