This is likely a HK/TW firmware problem?
In Feb 2020, when I bought my Note 10+ TW, I had a Starhub and M1 line. Both Starhub and M1 were successfully VOLTE enabled on both SIM card slots.
A few weeks later, there was an update that after that caused me to lose Starhub VOLTE.
Worse I also lost mobile connectivity as the APN was no longer picked up and I had to enter it manually. I had 4G connection but no VOLTE. Obviously this must be a Samsung HK/TW firmware issue that no longer provided support for Starhub VOLTE.
As the SIM card and my Starhub line was working fine when I switched back to the previous firmware.
There was no problem with M1 and I subsequently ported over to Singtel later.
I assumed that it was a firmware problem that would be eventually fixed.
Bro DigDub, was the APN auto detected for Starhub SIM? It seems that they have dropped support for Starhub inadvertently (or otherwise).