isolate.py saves state to desk as a cache for the next run. Supposedly, the presence of the cache should not affect final results.

However, it looks like some state is carried over. e.g. config_variables are added to the current run from the cache:
https://cs.chromium.org/chromium/infra/luci/client/isolate.py?type=cs&q=isolate.py&sq=package:chromium&g=0&l=501

I haven't been able to test this because of this bug:
https://bugs.chromium.org/p/chromium/issues/detail?id=873295

maruel@ states that this was left-over from GYP support and should be removed.