I've seen similar during own tests: phones connect to circuit-switched, but my
SGSN being broken caused a packet-switched failure, and the phone would rather
try another operator first. One way was to switch off mobile data on the phone.

Similarly, I have an iPhone 5c that simply will NOT stay on 4G/LTE and then goes into locked down mode until toggling aeroplane mode.

Then it will sometimes camp on 2G, but if it goes to 4G, shortly after going idle, (for example, if I ping it it'll stay) it will lock down again - I assume it is going off looking for VoLTE or something else on another carrier and getting told to shut down, but who knows...

When I switch the MNC/MNC on the SIM card to 001/01, I see an option on the phone to disable Volte and Voice over Wi-Fi. When I do disable Volte and Voice over Wi-Fi, the 2G disassociation issue goes away.

Availability of those options is due to the "carrier config", which is part of Android OS, IIUC, supplied to Goo by the industry, and subsequently part of AOSP.

But seeing as how this is a pixel, you can use an "app" called Pixel IMS, along with shizuku, (so you don't need actual "root" or other mods) to set the carrier config to whatever you like; enable/disable IMS reg with VoLTE/VoWifi. This will reset on OS update BTW - I use GrapheneOS, of course, (hard to think of a reason not to use Graphene if you are lucky enough to have a pixel!) so OS updates are rather regular.

:-) You can even decide whether "4g" should be called "4g" or "LTE". wow..

One /could/ use that app with any SIM card in the phone and modify the config for your carrier. (In Mexico, my Google Pixel actually works fine with VoLTE on Altan Networks, after changing the carrier config. (ok, yeah, as far as I can see, and I cannot see network side.) 

Current Pixel Models are now being sold in shops in late 2025, so maybe there's official support for VoLTE on 334140 now. I understand by law you can't sell any handset in Mexico that is not compatible with that network.

Anecdote: Altan is somewhat unusual in that it does not run any CS network. (I don't know where else this may be the case?) and originally they got around this by having a "Voice" app that runs on non VoLTE phones. 

I heard bad things about the performance of this app, but never used it. I guess it is essentially some kind of SIP VoIP thing with IMS compatibility; a "user space" app that suffers from all the imaginable battery drain and availability issues.

Since political changes and the state rescuing Altan from bankruptcy, other carriers are now obligated to allow "roaming" on their networks, but if one were to lock their phone onto another carrier's 2G network for CS service, the SIM will eventually be barred. It won't CSFB from Altan LTE to another carrier 2G of course, but incoming CS will work if already camped on 2G. At some point I guess, there won't be many non VoLTE phones around.