Tabs get automatically moved to a specific panel when reopened in a container

[Qhilm]

Troubleshooting done:

• Panel in question (right most panel in the window) has zero rules for automatically assigning tabs to it
• Tested with multiple other panels and multiple containers, it's always the same.

Context: this issue started last weekend and was not present before. Last weekend I did:

• Enabled firefox sync to a new device
• Disabled Sidebery, enabled ff native vertical tabs to test it, then disabled them again and reenabled sidebery.

Any idea? Sounds like corrupted config somewhere to me.