As you say, if Openreach came out then unless it was erroring at that particular time, they would have a hard job finding out what the cause was.
TBF it wouldn't bother me too much if the DLM didnt penalise me. As you can see I'd sat nicely for several weeks at 80Mb, SNRm not budging.
Then something will set it off (rather the daily spike will). I notice before DLM does so I take action and all calms down again. The next day DLM catches up and my speed drops to ~67Mbps and applies interleaving.
The only reason I can think of why I escaped DLM this time is although the Errors were totally unrelated to the Thunderstorms and just outside the time frame, DLM must have classed it as a Wide Area Event. My line is UG all the way, so is not as badly affected by storms as some other lines.
I would also love to know what causes that daily spike on quite a lot of lines even though it may not be as severe as mine is.