I believe the pinout is incorrect regarding D8 and D9 on the Tracker One Expansion page. The SOM page has TX being D8 while the expansion page has TX being D9. I believe that on the expansion page just the D8 and D9 labels should be swapped.
https://docs.particle.io/tutorials/asset-tracking/tracker-one-expansion/
https://docs.particle.io/datasheets/asset-tracking/tracker-som-datasheet/