I believe automatic closing of issues behavior can also occur when any of the commits have a “Closes issue …” in their commit message.
See this reference.
Because you have a large quantity of commits, this may be a little tedious/require a script to look through all of the 903 commits.
As a side note, I recommend looking at Gitlab MR Best Practices.
- Take one thing at a time and ship the smallest changes possible. By doing so, you’ll have faster reviews and your changes will be less prone to errors.