Hi there Max,
Thanks for getting in touch with us! I can help you here 🙂
First of allet me say that although converting posts to events using a plugin, was a good idea I wouldn’t expect that to be fully compatible with how our ‘Events’ post type works. So this is probably caused by this incompatibility.
I guess you’ll need to manually edit those entries and re-create the venue information as ‘Venues’ (WP-Admin > Events > Venues) and link them again from the event details. This is totally an edge case and although I’d love to help you getting things right, let me anticipate that the support we can give on this is fairly limited!
Please let me know if the above comments help,
Best,
Nico