I wonder if it's because the hose dia. is too small? We've had that problem with our Coleman Roadtrip grill which we run directly off the low pressure line under our TT (removed regulator at the grill) when we had a 20' 1/4" hose. I changed out the hose for a 3/8" hose of the same length and it works better.
I used to notice that when I first started the grill with 2 burners running at full heat, the flame was blue and strong. Then the flames gradually slowed and turned yellow on the tips. I'm no expert on propane but I'm guessing that the 1/4" hose was charged to the full low pressure level at first, then couldn't keep delivering that pressure at max. flow to the grill.