The MMR's official explanation is a "scam"

That’s a pretty good source thanks. Therefore I WAS ABSOLUTELY RIGHT the MMR got raised higher than the tanking l did to it at the d5 floor.

However notice some subtleties; they leave vague how they calculate the soft reset (it’s probably the MAX MMR you had); also they do not exactly say if TEMPORARILY the MMR drops during the tanking at a rank floor (it obviously does).

Yeah it pretty much confirms your soft reset theory. However notice they leave out how exactly they calculate the “true” MMR for the reset (it’s probably the MAX MMR of the previous month or even more months or maybe an average of maxes or other similar statistical metric).

Also they do not mention if TEMPORARILY the MMR tanks after too many loses (but it most probably does otherwise we wouldn’t be talking about a reset).