Since I upgraded to 21.04, my auto-hide dock (at the left of the screen) remains hidden until I switch its position to the right and back to the left. After that it works as expected (i.e. it appears when the mouse goes to the left edge of the screen).

EDIT after @vanadium's comment:

Normally the auto-hidden dock should become visible in its chosen position (left, below or right) when the mouse cursor comes to this edge of the screen. But for me, after starting up the computer and logging in, this does not work. The dock does not appear.

I have to change anything in the dock settings (e.g. change the location or switching the auto-hide feature off and on) to make it work correctly for the rest of the session.

PS: Today it worked from the beginning without any intervention. A mysterious case of self-healing?

PPS (one day after the PS): It happened again today. Perhaps a question of timing during startup? How can I delay the start of the dock?