I think this is likely because of disabling of NetDelayableH2AndQuicRequests, 
as the before change numbers for Control3 match the post-spike:
https://uma.googleplex.com/timeline_v2?sid=6536320989d1709cca0888280074d5c3

.. while others like Enabled3, EnabledHead3, and Yielding3 are better than the pre-numbers, with the pre-numbers for aggregate being vaguely near the average.

For some reason I can't seem to get a graph for Default, which seems to be the post-change config, though.

That's the kind of change I'd expect. Good catch.