A brief history of this Pixel 8: after buying it, the first thing I did was add an eSIM using the stock OS, from provider Tello in the USA. After that, I installed GrapheneOS and have been using it without issue for a few months. I never installed Google Play Services (nor any proprietary software whatsoever) under the Owner profile (only a secondary user); since the eSIM was added using the stock OS, this was never any kind of problem.

More recently, I traveled to Europe and bought a physical SIM card from Vodafone while there. I powered off the phone, inserted the SIM, and powered it back on. After boot I was surprised to see only one SIM card available, and even more surprised that the 'eSIM support' toggle had toggled itself off altogether. I sighed and ignored this during my few days in Europe, where the physical Vodafone SIM worked without issue. Upon return to the US, I was unable to get the original eSIM to work again:

To my relief, enabling the 'eSIM support' toggle seemingly worked. It demanded a reboot (as expected), and stayed on afterwards. (Why it ever became disabled is still unclear to me.) After this, the eSIM was once again listed in the list of SIMs, albeit in the disabled state. If the Vodaphone SIM is also disabled, then attempting to enable the eSIM fails quickly and quietly: the toggle changes to the enabled state, then just changes back to disabled a second or so later without any explanation. If, on the other hand, the Vodaphone SIM is enabled, then first I get the screens where I have to name the SIMs and select the eSIM for all three of data, text and talk. It then displays the "Switching to ... " message for quite a few seconds -- unlike the first case -- but eventually also fails silently, with the SIM's toggle resetting itself to the disabled state. I tried both of these approaches multiple times, with identical results.

Eventually, I tried installing the sandboxed Google Play Services in the owner profile (and choosing to leave Network permission enabled while doing so). This didn't change anything; both failure modes behaved exactly the same as before. Physically removing the Vodafone SIM card from the slot didn't change anything, either: just like when that SIM was disabled in Settings, attempting to enable the eSIM quickly and silently failed, and the network indicator was altogether absent, none of that "emergency only" stuff.

Finally, I requested a new eSIM from Tello (for the same account and phone number). Since I had already caved and installed Google Play Services, I was able to add it from within GrapheneOS this time, and this worked perfectly with no unexpected extra steps (permissions or otherwise). At this point I erased the original eSIM and uninstalled Google Play Services; the eSIM continues to work fine even after several reboots. I have not attempted reinserting any physical SIM card since then.

This seems like neither expected behavior nor a known issue, as far as I could find, anyway. So there we have it :) if anyone is interested in debugging this, I bet it will reproduce if I try to insert the physical card again...