Gas gauge issue

[Georgaina]

New issue….

The other day I filled up with gas.  The pump shut off at full automatically.  Got in my truck, started it, and the gas gauge was only at 3/4 and the miles remaining to empty were at 359, lower than usual for a full tank.  Hmmm. 
By the time I drove 20 miles home, the gas gauge had gone up to full and I had 393 miles left till empty.  Normal. 
Got in the next morning to go to work and I had 299 miles to go and less than 3/4 of a tank.  Nobody stole my truck and drove it during the night.  
Went to work again.  Miles left got al the way up to 384.   Got in after work, 276 miles to empty.  Drove a bit and was idling at a red light.   Noticed my miles left to empty were actively flipping, going way down then way up. Happened every time I was stopped. 
I get that the miles left feature depends on driving history.  It also moves the gauge dial. 
I made a service appointment for a week from tomorrow.  I have pics and video. This is a new issue, I’ve had my truck for 2 years and this has never happened. 
Anyone else?  Any idea what’s wrong?
2019 TB, 29200 miles. 

 

[Transient]

Sounds like a fuel pump issue via the float.  Just a random guess because I'm not very good at diagnosing over the phone.