I don’t think that they can realistically fix all before migration. So, probably issues will have to be reviewed and migrate accordingly. “Spring cleaning” style.
Otherwise, Github/Gitlab seems to rather divisive issue, way more heated than I expected. In the end of the day, it is good to consolidate on single platform and I see benefit in offloading this to some service provider. So, all in all, whatever destination you choose, looking forward to it.