It's an old bug making a reappearance. It seems that the delves are taking players out of the Cyrodiil population and so its causing issues when they try to go back into Cyrodiil, particularly if Cyro is pop-locked.
If /stuck and dying/respawn don't work, you should be able to leave by queuing for a non-poplocked campaign. That won't help with getting out to play in Cyrodiil, but it should at least get you out of the delve.
I logged out to make the OP and when I logged back in it sent me to Stormhaven. Was worried it was going to put me back in the delve as character select screen had me in there yet. Had never seen it before, was just weird. Also didn't know if it was a known issue or worth mentioning. Thanks for the response.
I used to encounter this bug when grinding CP in Cyro delves way back in 2.0. I was always able to make it back into Cyrodiil by trying to exit repeatedly.
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I tried probably 20 times. 3 or 4 times it went into a longer load screen like it was going to work but it didn't. After logging out and coming back into Wayrest I immediately entered Cyrodiil without a queue. Not sure if it still had me on the list as it was about 5 minutes between logging and re-entering Cyrodiil, or if the population just happened to dip beneath the lock limit. It was only around 3 or 4 queue when I originally entered so it was close either way.