It seems that is a bug, either in Qt Creator (not generating the correct escaped sequence) or in PySide (pyside6-uic doesn't generate the correct escaped sequence for QCoreApplication.translate() or QCoreApplication.translate() doesn't accept 16bit escape sequences).

A bug that seems to be related (QTBUG-122975, as pointed by @musicamante in the discussion) seems to be open since 2024.

As a workaround, for the time being, if your app doesn't need translation, you can deselect the translatable property in the QAbstractButton properties.