Direct steam injection heating

I'm presently working with two identical wiped thin film evaporators that increase concentration of gelatin. 180 PSI direct steam injection is used to heat the gelatin from 120F to 285F on it's way to the evaporators. Both streams are pumped from identical progressing cavity PD pumps through identical 2" piping. The mixing on both streams is done at what looks like a standard 2" piping tee. Steam control valves exist upstream of the tees on both streams and operate properly to regulate steam flow to bring the temperatures to 285F. Process conditions call for the evaporators to go on water before gelatin is introduced and herein lies my problem. One of the streams experiences cavitation in the piping when on water. I put the feed rate on the affected loop in manual and adjusted output to the PD pump to get approx. 7200 LBS/HR. When the temp loop output is adjusted to add a small amount of steam the flow rate drops to 5600 to 5800 LBS/HR with the same pump speed. Any additional increase in steam and the metered flow becomes erratic (sometimes dropping to zero) and cavitation occurs.

Is the PD pump stator worn enough to where the additional discharge head caused by the steam injection is causing the pump to slip? That was my initial thought. But I can't help but think that the ordinary looking tees aren't all that ordinary. Steam runs straight through and gelatin is introduced at the right angle. Has anybody seen a type of tee where there are internals that direct both steam and liquid in the right direction? There are no external marking on the tee that I can see. Operators tell me "that one has always done that". I can't help but think that ordinary looking tee is installed backwards.