In the tracker-one datasheet, it has this to say about orientation
As the GNSS antenna faces the top of the case, you also want the top of the case facing the sky to
the greatest extent possible. You will likely be be unable to get a GNSS lock with the top facing
However, if I am only using the cellular radio, I would think a vertical orientation with the M8 connector down would be better. An APP note from Laird says this is their preferred cell antenna orientation. Long axis of the paste on antenna should be vertical.
The cellular antenna will function in either direction.
The Monitor One has a slightly different cellular antenna, but it is quite similar and located in the same position as the Tracker One. Because the Monitor One GNSS antenna is mounted at an angle, it can be used back down (like the Tracker One), or vertically with the connectors facing down (like you want to use the Tracker One with GNSS).
I agree all orientations will work, but Laird strongly suggest the vertical orientation. I a previous job, reorienting the cell antenna from horizontal to vertical improved my reception.
I'm speculating that Particle orientation was based on a good compromised if you are using all antennas, but not optimized if you are only using the cellular antenna.