I usually try and mash for high fermentability (and make adequately sized starters etc), but my attenuation almost never leaves the 70-75% range. (Saisons are an exception, but even there is see 85-90% rather than the 95%+ often reported.)
And yes, I have checked my thermometers as well as my hydrometer.
In an effort to increase attenuation, I had started to mash in at 60C and then raise the temp only after 10 minutes or so, to ensure beta-amylase would not denature before nutrients and enzymes were distributed in the wort etc.
Based on an article I encountered very recently (
Optimization of Beer Brewing by Monitoring α-Amylase and β-Amylase Activities during Mashing), that line of thinking appears to be correct, but the temperature isn't: to actually preserve beta-amylase, you need to mash in below 55C. So it seems like what I was doing was actually slowly kill off the beta-amylase at a temperature below its optimum range and before gelatinisation of starches.
Anyhow, to bring it back to *your* problem, if you want to call it such: if you mash in high, it shouldn't make a ton of difference if the temperature drops a bit after a while (like half an hour). Beta-amylase denatures very quickly at 69C.
And for the whole strike-water-temp thing: it seems weird from a perspective of physics, but I have never observed that temperature drop at dough-in. Not when recirculating with the Grainfather and not when stirring manually in a pot. I'm admittedly puzzled as to what I might be doing wrong (or at least different from everyone else).