It happens when: - opening a file via right-click > Open with > XYZ - moving a file via drag and drop to the Thunar window - ... I recall that I have done a test with a compiled version of 1.8.0(?), and the same bug was present. Xubuntu 19.10 (dev)
The bug does not occur when triggering the "Open with" action via menubar > File.
It is not happening anymore. Tested with 1.8.7.