Double Commander was upgraded to v0.5.9 when I d-u on 04/19/2014. I have locked tabs in Double Commander. When I click on another tab, the previous tab disappears. This appears to only be an issue under Sid/Siduction, since I'm running the same program under Debian Stable without an issue, although I'm running it from a static portable build.
Thankfully, the tabs don't disappear permanently. If I close and re-open Double Commander, the tabs are all there again.