I used a freshly-caught Abra's Teleport in Ultra Space, and it sent me to a location in the Overworld (x=-927, z=82) that was not my Healer (x=-422, z=387). It teleported me a few blocks underground as well, so I had to dig straight up to not suffocate.
When I broke the surface of the Overworld, there was a Nether Portal next to me. I hadn't built one in that world yet. I went through it, and it sent me to the Nether. I then used Teleport in the Nether, and it sent me to my Healer, meaning my Healer was definitely working as a Teleport anchor.
My hypothesis is that there's no handler for using Teleport to change dimensions in Ultra Space, making the game default to what it does for the Nether. This would explain my strange Overworld coordinates as corresponding to my location in Ultra Space, as well as the Nether Portal being generated near me. Should be easy enough to fix.
Keep up the great work, dev team!