BITRISE_GIT_MESSAGE env var is read from the commit message API param. From the linked docs:
commit message - or the message you specified if you triggered the build manually. This env var is mapped to
commit message in the API.
So this env var is basically holds the value you pass as the commit message via the API. In the past it was possible to overwrite it via custom env vars in the api call but that was never our intention - to set this env var’s value please use the related API parameter (
commit_message) instead, as the
BITRISE_GIT_MESSAGE variable will be set to that value.
The web UI also only reads the value specified via the API endpoint’s
commit_message parameter, it does not read custom env vars, only the documented API parameters.
I hope this helps/explains the situation, let us know if you’d have any questions!