I do not know if me only facing this issue or not. Previous version Ubuntu 20.04 always show Plank Deck either after system start / restart or after Logout. No matter what my laptop's condition, connector to power or using it battery.
This version when my laptop is not connected to power, rely only on batter, it boots slow and does not show the Plank Deck. I have to let it appear by press menu and searching for it.
Today when my laptop is connected to power supply, it did not show the Plank Deck. I had to press the menu and then choose Plank to reveal it self
I have a similar problem, but my laptop does not have a battery (maybe this is the relation to the issue). Once I configure me desktop with Mate-tweak and make the necessary adjustments to have it as needed, Plank will show on start-up for a few days, and then, nothing. As Salah indicates, I have to start it from menu.
Are you by any chance using a dock with external monitors?
I have found the issue that the dock doesn't reposition when I undock (and the primary screen changes as a result). The dock stays in the same position as before, and since my external monitor is bigger than the laptop display, the dock is simply out of viewport.
Maybe a wild idea but could you try to give plank a startup delay of 10 seconds ? There might be a racecondition where starting plank too early results in a fail.
I just realized that booting on battery is probably slower than on AC.