Maximum length of an environment variable

I'm using Azure Function App hosted on an S1 Linux plan and I've noticed that whenever I set (through Azure Portal configuration ui) environment variable which is longer than 53 characters then it's getting trimmed.

Example:
Environment variable name: `Section1__Subsection1__Subsection1OfSection1__ConnectionString`

Portal allows to set such environment variable and after saving it still displays full length of it. But when I go to Kudu and open an appsettings view I can see it's trimmed to `Section1__Subsection1__Subsection1OfSection1__Connect`.

Works fine when app is hosted on Windows plan.

Couldn't find it documented anywhere.