A blank task bar icon usually happens when the file it was pointed to has either been uninstalled or is now located on another path. It can also happen when the icon has changed design due to an updated file and the icon cache hasn't picked up on it.
Just right-click the blank icon and choose "unpin from taskbar." I would also remove the "hidden" one too just to be sure you're starting clean. Then, simply locate the MusicBee.exe file wherever you installed 3.3, right-click it, and choose "create shortcut" (NOT "pin to taskbar"). Then, after maybe renaming the shortcut to something friendlier like "MusicBee 3.3", right-click the shortcut you made and choose "Pin to taskbar." Done.