Both Unity8 and QtMir are excused. On the day of the regression, unity8 was unchanged and qtmir changed but not in logic.
This leaves only Mir. On the day of the regression Mir was upgraded from 0.19.3 to 0.20.0. And looking at the source code, a large amount of code changed in Mir's buffering/GL logic between those releases.
Both Unity8 and QtMir are excused. On the day of the regression, unity8 was unchanged and qtmir changed but not in logic.
This leaves only Mir. On the day of the regression Mir was upgraded from 0.19.3 to 0.20.0. And looking at the source code, a large amount of code changed in Mir's buffering/GL logic between those releases.