How can phased rollouts be debugged in Chrome? Is there a screen which shows "you are in rollout group A" or B ?
Same #Chrome Version 70.0.3538.102 (Official Build) (64-Bit) version, for me the cert does not work (first screenshot), for my colleague it still does (second screenshot).
The About Screen shows the same version.
Unfortunately, this is (intentionally) difficult.
Since you know exactly what feature is causing trouble, you can force it with either --enable-features=LegacySymantecPKI or --disable-features=LegacySymantecPKI.
If you want to roll the dice, that's --reset-variation-state, and you can view the current variations at chrome://version. Since the variation state only has 13 bits of entropy1, there will be a large overlap in which variations are toggled together, so you'll have a hard time determining which hashed feature in this list corresponds to LegacySymantecPKI, Disabled, and Enabled.
There's a feature request to show the unhashed field trial names: https://crbug.com/653634.
There's a design doc for the hashing here: https://cs.chromium.org/chromium/src/components/variations/variations_murmur_hash.h
