I saw an issue with a similar name, and it is supposedly fixed, however I am still getting the same issue. Normal Barrels seem to work, but so far Bamboo, Acacia, and Dark Oak all delete any items placed inside immediately upon exiting the barrel.

I am using Blockus version 1.1.6 according to the jar name.

Apparently wasn't using latest version. Trying it out and will reopen if I find it still causing an issue.