Tested on Groovy. I can confirm reported issue albeit not for _all_ flatpaks.
I tested GitKraken, FreeTube, Warpinator and Agenda flatpaks. Of them, FreeTube showed icon properly both in task switcher and window list (bottom bar) but other three did _not_.
Observed same issue on LinuxMint MATE as well. Haven't confirmed on other DE than MATE or non-Ubuntu distro.
Tested on Groovy. I can confirm reported issue albeit not for _all_ flatpaks.
I tested GitKraken, FreeTube, Warpinator and Agenda flatpaks. Of them, FreeTube showed icon properly both in task switcher and window list (bottom bar) but other three did _not_.
Observed same issue on LinuxMint MATE as well. Haven't confirmed on other DE than MATE or non-Ubuntu distro.