I’ve seen fatwood do this. The heartwood core was jostled loose so it had formed a very strong seal with the sap around the core but as soon as I started splitting it, more it dried out it really just fell out. In fact I still have part of the round, some of of the most concentrated fatwood I’ve seen. I had another split that did this but can’t remember what wood it was..
I had to laugh last night when I was putting wood into the stove. One looked just about like yours JDU