Missed target gravity by 10 pts. Strange burner mishap to blame?

I brewed up a 5.5 gal blonde on Saturday morning. I used Deathbrewer's partial mash biab technique: https://www.homebrewtalk.com/f39/easy-partial-mash-brewing-pics-75231/

Here's the recipe:

4lb Belgian 2-Row
.5lb Acidulated
.5lb Vienna
.5lb White Wheat Malt
3.3lb Pilsen light LME
1lb light DME
1oz Styrian Goldings @ 60min
.75oz Sterling @ 15 min

At my usual 60% efficiency I should have been at 1.050 (per BeerSmith). The reading I took, going into the fermenter, was 1.040...around 30% efficiency (again, per BeerSmith).

Not really sure what happened but I have a theory. I bought a propane burner this weekend, this was my first time brewing outside of the kitchen. So I mashed in, trying to hit a target temp of 156. I landed at 150 and rather than RDWHAHB I decided to fire up the burner for a few seconds. I lit the burner and watched the thermometer. It didn't really move. I waited for about a minute, watching it not move. So I grabbed a spoon and stirred, the thermometer immediatly spiked up to 200 then fell immediately back down to about 155. Seems like the therm. was stuck in a cold spot before the stirring. I turned off the heat covered it up and started the timer on the mash.

Did the blast of heat render a portion of the grain unfermentable? I was thinking I might have nuked some enzymes there.

Anyways, fermentation took off like a rocket. Pitched a 4L starter of Brett C. and Brett B. with a packet each of lacto and pedio. Pitched warm, about 76-78, but it was rocking in two hours and yesterday it was bubbling harder than just about any beer I've ever made. So I guess it's okay.

Anyways, thoughts?

Even if you got some extra long-chain sugars from the temp spike, which you probably didn't, those sugars would still factor into the OG reading. Gravity is a measure of liquid density relative to water, so anything more dense than water factors into the gravity reading. If anything, that quick spike would serve to act like a mashout and actually increase your gravity.

Efficiency issues like this typically boil down (<--PUN!!) to either problems with the grain crush, false readings from not having a uniform density, i.e., not stirring the everliving crap out of the wort before taking a reading, or using too much top off water post-boil.

Efficiency issues like this typically boil down (<--PUN!!) to either problems with the grain crush, false readings from not having a uniform density, i.e., not stirring the everliving crap out of the wort before taking a reading, or using too much top off water post-boil.

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Certainly a possibility I suppose. Since I don't do full boils, I try to be extra thorough with my readings. I did top off with 1.5 gallons, which is fairly normal for my brews. I usually shake the mother-lovin' hell out of the carboy before taking my sample...but I didn't do it as vigorously this time. My rationale was to keep the oxygen level on the low side, for the sake of the Brett.

If anything, that quick spike would serve to act like a mashout and actually increase your gravity.

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A mashout on the way in...hmmm.