I can explain the reason fairly easily.
You bought a Premier with 2 rail/ 3 rail trucks!!!!
Proto-Scale 3-2 3-Rail/2-Rail Conversion Capable
Why does that matter? Well it means the wheels and axles are isolated to run on 2 rail track, and yes, there is a switch and wiring to hopefully connect those isolated wheels when set to 3 rail mode.
You "doubled down" on the problem using Gargraves track- known to be isolated outer rails. Atlas and some other tracks are the same way.
This combination- along with typically the owner not wiring the track with feeders to connect and bond the outer 2 rails frequently implies you likely have sections of track where only one outer rail is bonded to common.
So what happens is- you have an engine that may not be picking up common from all the wheels on a truck and specifically one sided. You then also have a track that by default does not connect the outer two rails and requires you the owner to account for this when wiring the track.
This double combination of less than ideal electrical pickup situation on the loco and less than ideal track situation with wiring- can result in this exact problem.
It's recommended to move the 2/3 rail switch back and forth several times to clean the contacts to see if that improves the pickup at the wheels. Otherwise many users bypass this switch.
Obviously- more work, but your track system needs the outer rails bonded for best electrical continuity and running of all trains.
