Just to be clear, for most anyone with low-thrust measurement experience, thermal drift had been suspect nr 1 from the moment Eagleworks published their work. Here is a comment I made back at that time:
Lol at the lone reply you received. You called it and that's what you got. I hope we can learn to look at all points from more perspectives in the future.
This post shows how hard it was for scientists to prove it's bollocks. And the above comment shows how the general population doesn't want their hopes dashed.
It really was clear at the time. Even then people pointed out that the measurements didn't behave in the way you'd expect if it actually worked (e.g. thrust didn't correlate with power).
There were plenty of people saying "But imagine if it did work! We could get to other stars in a few years!" but you always get crazy optimists that ignore all the evidence with stuff like this (c.f. solar roadways).
It is what is mentioned in this fragment of the article:
"When power flows into the EmDrive, the engine warms up. This also causes the fastening elements on the scale to warp, causing the scale to move to a new zero point"
Basically, thermal expansion of the thruster and the measurement device causes the rest position of the thrust balance to drift with time, which is hard to differentiate from the actual thrust generated. This is most critical when the thrust-to-weight ratio is low, as is the case here.
https://news.ycombinator.com/item?id=12962579
But then, nobody had Tajmar's perseverance; kudos to his team!