I did receive an answer from Small Giant’s support staff to the issue about the uneven number of players in the battlefield I opened a ticket about before writing the original post.
I think it could be useful to share it here so that the community is aware and may check if the solution / explanation could fit their case too.
Support staff had carefully checked the logs then verified that at the time of the matchmaking we were 22, not 21. Then, according to their answer, a player left after the preparation phase.
I checked the membership time of all our mates and no one of our allies left then rejoined the alliance from long time.
However, being an open ally, many players repeatedly join then leaves after a few time, maybe looking for titans to defeat in order to complete the valor path (this is our best supposition for that behavior).
So we thought that the incident might have happened due to an unlucky timing: one of those “visitors” joined the alliance then left during or nearby the matchmaking phase.
Now we set the alliance to invite only and won’t accept players when the matchmaking is about to start.
This could be a suggestion to prevent similar issues from happening again if our deduction is correct.
I hope that these informations could be useful to someone else or help finding a better explanation for such behavior.