I have been having this problem since prior to the Ubuntu 20.04 upgrade, so I don't think it's release-dependent.
When my laptop (Lenovo Yoga 720) comes back from sleep mode, many of the dock icons on the top panel are missing. The applications have not stopped running, but the icons don't come back.
The workaround I've found is to log out and log back in.
Here is a picture of the top panel before sleep:
Top panel before sleep
Here is the top panel after sleep:
Top panel after sleep
