I would move this thread to the Weather Bug forum and rename it “Native Support of All Cloud Types functionality does not work”. See the weather discovery serie published when FS2020 was launched:
Feature Discovery Series Episode 2: Weather - YouTube
Not only cumulonimbus type is not represented right but many other cloud types are missing. Also in that video you can realize that many other advertised weather functionalities are not working or are not implemented.
There is also this thread in the weather bug forum that talk about the issue with cloud types:
METAR keeps disrupting the weather/ bugged weather/Cumulus/CB clouds only/no medium to high cloud coverage
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