Damage to Plum Branch

I have an Emerald Beaut Plum which has some peculiar damage to one of the upper branches. I was able to prune it off without any harm to the tree, What could this be? The hole seems filled with sap.

Here’s a cross section view, no bugs seen

Bacterial canker?

Hmm…I pruned off that branch 6 inches below that hole. What do I need to look for now?