Engine is hard to start and idles poorly at 1/4 tank reading

[68ls1]

Hope someone may have a reason on this. When fuel gauge is showing 1/4 tank or less, Blazer become stubborn to start and idles poorly. I currently don't have truck to look at.
Mechanic who did brake, suspension and some mechanical repair commented on this, and put more fuel in.
When I first brought it home, there was around 1/2 tanks and started easily etc. As we ran fuel down while testing things including temperature gauge replacement, once fuel gauge got to around 1/4 it again became hard to start.
I think put around 10 gallons in to Blazer for my trip to body shop, and it again started easily on all occasions.
After the drive it was down to showing 1/4 tank, but driving normally. As soon as I pulled up, put trans in park it stalled, like out of fuel.

Fuel gauge could be out, but seemed to operate perfectly on trip, original mechanic had put some fuel in it and was still acting poorly. Put new fuel pump on but situation remained, put more fuel in, problem gone.

The truck does have the remnants of an auxiliary tank, so fuel lines are a bit of a mess. This is planned to be fixed.

Any other thoughts on this, why fuel feed would diminish when tank gets low?