Something is not adding up here. If your HLT is hotter than (or at least equal to) your mash, then your mash temp should be going up (or staying the same), not down. During this last session, what was the ambient air temperature? When it is cold I wrap my mash tun (S/S pot with false bottom) with a blanket, during the summer it is not needed. I use my HLT for my HERMS as well and as soon as I am satisfied that mashout is complete, I sparge from my HLT. I am using my return manifold to try and keep an inch or so of sparge water on top of the mash. Are you using a "whirly" sparge arm? If so you will get heat loss from that. If you are using a whirly type sparge arm, trying raising your HLT temperature a few degrees to compensate.
On my setup I gravity feed from the HLT to the mash and use a pump to pump from the Mash Tun to the boil kettle. I really don't know the exact rate, but I match the pump flow with the sparge flow to try and maintain the inch or so above the mash.
Thanks for your inputs.
Ambient temp should have not been an issue. On the day it was around 25C. I am able to keep my oblong cooler closed for the duration of the mash & sparge, made possible by a height variable copper tube input (mash/sparge) in the cooler lid and provision for using a long stick thermometer through the lid. I only stir at dough-in, not at any other time. So heat loss in those regards should be at a minimum.
See my reply to Raven 19 about how I propose to try and overcome the problem.
Would be glad to hear your views on that proposal.