So as you can see in the video, liquid blocks disappears when u fill them in using keep, hollow, destroy, outline, and replace I would have made the video longer but It only goes up to 10 megabities. It does it for all of those settings. I'm very sure this is a bug because it doesn't make sence for lava or water to just disappear like that, it's very unusual. I wouldn't be so upset if it weren't for that lava is not like water, they don't  be still unless u put it in each individual block slot. Very weird bug, I use fill alot, and It sucks that the other options for them just makes them disappear for some reason. Not to sure why, but it makes tedious placements very annoying and and I can't by pass it anymore, plz check this out and let me know if this is a bug or not, cause I can't really think of any reason it isn't. I'll keep reporting any command bugs if necessary. Thank you!