I have two mono tracks that I want to merge in one stereo track.
Probably not. You have two mono tracks you want to mix down to two-track mono. That’s different.
Select the whole thing by double clicking anywhere in the blue waves and…
Tracks > Stereo Track to Mono.
That will mix the left and right together and produce a “real” mono track. If you did nothing else right there but File > Export, you would have a single track that will play to both stereo speakers.
If that bothers you, Select the whole thing and Edit > Duplicate. Using the drop-down to the left of the top track > Make Stereo Track.
That will give you two-track mono.
It’s not a bug and yes, it’s a little out of the way, but then what you did isn’t particularly normal. You can also get that by duplicating each of the mono tracks at the beginning and make each one into a “stereo” show. When Audacity exports the work, it will mix the two stereo tracks to one stereo track. That’s dangerous because you could get overload if both tracks got loud at the same time.
No, it’s not a bug. It is behaving as designed and as described in the manual: Audacity Manual
Note that there is no need for your first two steps.
If you want Track B to be the left channel, simply drag the tracks into the correct order by clicking on the track control panel (left end of the track - taking care not to click on the sliders or buttons) and drag up or down.
There is also an overview of splitting and joining stereo tracks in the manual: Audacity Manual