True – not even Apple gets this right, being otherwise pretty good about timely software updates.
My MVNO started being recognized incorrectly as being part of another network years ago, and apparently their engineers have been unable to reach anyone within Apple about the matter and get theem to apply what should be a trivial fix in their IMSI mapping table.
Apple has never been good in this department, according to the folks I've talked to within mobile carriers.
Even Google, a nearly $2T company, operates an MVNO ("Google Fi") that does not work properly on the iPhone, because Apple has not added Fi's GID to the correct list.
Jailbroken users can (in theory) add Fi's GID to T-Mobile's "carrier bundle," and things like Wi-Fi calling, 5G connectivity, etc will work instantly.
Originally there was a technical reason for that: Google Fi uses several carriers, not just T-Mobile, to increase coverage, and at the time preliminary iPhone support first came out for Google Fi, iPhones didn't have support for connecting to two carriers at once and switching back and forth between them based on signal.
So right now, someone on Google Fi with an iPhone and someone on Google Fi with an Android phone can be standing next to each other, and the Android phone could have great signal while the iPhone has zero signal because T-Mobile has no coverage at that location.
I don't know if iPhones now support that; if they do, then there's no longer any technical reason iPhones couldn't have first-class support for Fi.
My MVNO started being recognized incorrectly as being part of another network years ago, and apparently their engineers have been unable to reach anyone within Apple about the matter and get theem to apply what should be a trivial fix in their IMSI mapping table.