Hi!
It seems the build version of Qt doesn't match the version installed with Plucky (6.8.3) which leads to the following error:
$ mcpelauncher-ui-qt
Loading gamepad mappings: /usr/share/mcpelauncher/gamecontrollerdb.txt
Loading gamepad mappings: /usr/share//mcpelauncher/gamecontrollerdb.txt
mcpelauncher-ui-qt: symbol lookup error: mcpelauncher-ui-qt: undefined symbol: close, version Qt_6
(and the ui doesn't load)