I'm undiagnosed, so grain of salt time, but one of the reasons I have been told (by family) that I'm not is that I AM very expressive. The hands move, the face moves, the voice inflects, the pitch changes, and the intonation fluxuates. Also, all the eye contact happens
However, it was my daughter who pointed out that this happens mostly when I'm out among people I don't know well, or when I am in 'charge' somewhere, like (OP said) when teaching in my 'special interest'. I become very animated. But at home, I become very autonomous. Very little fluctuation in volume/pitch/tone/etc. Minimal facial expression, unless negative (bad smell, angry, upset, etc).
So it could be a special interest kind of thing.
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Aspie Quiz: 148 ND/50 NT
AQ: 41 (AQ-10: 9) EQ: 17 SQ: 31 FQ: 44 RAADS-R: 178
ASD Diagnosed 4/22/2016