The Evaporation monitor routine is done in 4 phases and some actions depend on tank % fill and other factors.
You opened the tank to fill, so the routine has to begin again from some point.
The dash gage is micro processor controlled and does the gage update independently and is subject to the signals it gets from the float level etc.
What the level 3 has to do with it is anybodys guess at this point.
At the least, it upsets the drive cycle performance that has to start again from a cold engine, which is not the case at the gas pumps.

 

I have tuned my truck while filling up with gas, and the guage still works. I have filled the tank with the engine running and the guage tries to keep up with the actual level. (it's a little slow) But I do have a 97. The gas guage is one thing that actually never has malfunctioned on my vehicle.