I'm using the efi squashfs version of the x86-64 image version 21.02.1. After following the doc to resize the partition and filesystem to maximum, upon rebooting, my /boot is empty, and /etc/mstab contains no /dev/sda1 entry, but everything else seems to fine, I can connect to the internet without any issues.
What might cause /boot to not be auto-mounted after booting?
After some endeavor, I managed to locate the code that mounts /boot:
It seems /boot will only be mounted, at lease in my case, if the partition (the one containing the root filesystem) ID is ended with 02, and I can confirm it works by changing the partition ID manually.
I wonder what's the significance of 02?
When one follows the doc, the partition ID is bound to change. Whether it ends with 02 is totally by chance IMO.