Problem with MIDI since Ardour 3.4

Since having updated Ardour to version 3.4 I’m experiencing weird MIDI playback behavior:

Whenever I trigger a MIDI note
(e.g. by pressing one of the keys next to the piano roll) while the song is not playing, the note will never stop but be stuck in an infinite loop until I hit the play button.

Does Anyone have the same problem or any idea what might be causing this problem? This has never happened before in previos versions…