Hello @Gael , thanks for your report!
We suspect this could be related to a known behaviour that was brought up on this other topic. In short: when you put an instance of Symbol A (with custom overrides) inside Symbol B, swapping instances of Symbol B won’t consider the overrides in A as overrides anymore, and will reset them to what’s on the Source of A.
It’s a bit complex to reproduce and therefore not commonly seen, but we’re aware of this and something we have to tackle in the future.
This being said, this could be something completely different altogether, so we’d love to test it ourselves. Is there any possibility of having a copy of these three documents? I mean, the document that appears in your video, the external Library and the theme Library. This way, we can try to reproduce the issue and determine if this could be an issue in Copenhagen or the already known behaviour I described above.
If possible, please drop us an email to productsupport@sketch.com, including the URLs (and support access granted if these are in a workspace) or an attachment with the documents and we’ll take a closer look.
Thanks for reporting!