AFS has containers to install betas on the same partition, the main partition is not polluted (and the additional volume(s) can be removed at will). cfr. https://support.apple.com/en-us/HT208891
I'm not saying that it's "normal" not to be able to boot from external devices, I'm just answering your point
This is quite risky - previous beta updates (such as Big Sur) did break installing updates on older OS releases in the same APFS container, and some beta updates have in the past done incompatible APFS updates that broke the ability to boot old OSes.
I'm not saying that it's "normal" not to be able to boot from external devices, I'm just answering your point