I have enqueued a few tracks from a playlist (let's say playlist "A") and they do appear in the playback queue (the playlist assigned from the playback/"playback queue" main menu).

Now when I select any track to play in playlist "A" through the (right click) context menu, the selected track is played but the playback queue is wiped out and appears empty. [The same happens if i double click the track].

I probably have missed something [as usual], but I'm curious to know if this is the expected behaviour.

This is indeed expected behaviour. The idea was to make it behave similar to double-clicking on a playlist item without being restricted to the playlist.

Of course .... but what about the playback queue being wipped out?

The behavior is intended (specifying next track to play manually bypasses queue logic).