Your EVAP system is monitored by a pressure sensor that trips a CEL if the vacuum exceeds (or is less than) the normal range. It sounds like because of the EVAP system leak the light keeps tripping. The check here is called a “smoke test”. Smoke is piped into the void areas in the system and where the smoke leaks out is where the leak is.
This has no relationship to octane or to heavy rain. I still suspect you need a tuneup…and now suggest a smoke test.