I assume most BIAB brewers mash with their full volume of water for the entire brew.
Assuming this I have been heating my strike water regularly over 165 degrees.
Is it possible that, by doing this to achieve mash temperatures of 158, I am denaturing the enzymes of the malts before reaching a homogenized mash temperature?
My last three brews had efficiencies of 50, 65, 57 respectively. No consistency at all.
My ph of the last brew was 5.26
I double crushed my grain.
I squeeze a lot
I stir after 30 minutes into the mash.
When I did BIAB I was using typically about 7 or 7.25 gallons of strike water. I'd heat it to about 158 or 159.
Here are the things that affect mash temp: strike water temp, strike water volume, temperature of the crushed grain, amount of the crushed grain.
If you're having to go to 165, something's wrong. You don't say what your water volume is, nor the amount of grain nor where it was stored prior to doughing in, so hard to pinpoint some source of the problem without more info.
I learned over time that there were hidden places that temp was influenced. Once I kept my crushed grain in the garage overnight, and temps there dropped to 50. Guess what that did to my mash temp? Because the grain was cold, dropped it lower than I'd expected. Now, the grain is always kept in the house at room temp.
I also discovered that there was latent heat in my propane burner. I was using a Hellfire, and when I got the temp to the right point, I'd shut off the gas. Guess what? That burner was HOT; the kettle supports even turned red in the heat. When I turned off the propane, the hot burner assembly kept heating the water, ending up higher than I'd intended. I learned to stop heating a few degrees short of my target, expecting it to rise into that point.
These are all things that might be affecting your results. Assuming your thermometer is actually accurate, there's no way you should need to go to 165 degrees and, yes, that may be denaturing some of the enzymes before the temp drops enough.
Check out some of the above, let us know what you discover is the problem. Good luck!