I saw this too late (also this), but I also created an issue about this on GitHub:
https://github.com/bitrise-io/bitrise-webhooks/issues/81
I had dug a bit into this and saw that GitLab is sending all the required information to distinguish different actions, but Bitrise’s webhook for GitLab (last updated in 2017) doesn’t take this into account.
I was told the internal ticket is probably low-priority compared to other stuff, so if you want it fixed faster, feel free to open a pull request. 