do you think it's an issue of gitlab or of the transition?

As far I know Jira and GitLab issues initially can look kinda similar (depending on how Jira is used) but are very different in what works well with them so Jira to GitLab transition seems rather risky.

And while I haven't used GitLab issues a lot, GitHub issues are kinda terrible and Jira isn't better either (but bad in different ways; with github actions you can try to "fix" GitHub but in practice this doesn't work that well my experience and can't fix the most important issues)