Convert lines to fill delets lines?

Hey, so i'm finnally getting the hang of drawing with my adove animate cc 2021 package. One problem that I seem to be running into though is that after I use the pencil to draw my lines, then use "combine objects>union" I'm able to fill the spaces of my linework with the fill bucket. HOWEVER, sometimes, no matter how many lines I put down or how sure I am that they are connected, the fill bucket crosses these lines and fills way more than I wanted it to. I usually fix this problem by using the "convert lines to fill tool" which usually gives me better results. yet sometimes when i use "convert lines to fill" It Erases several of the lines that the painbucket was ignoring too. This leaves me with a big chuck of linework just deleted for some reason. What Is happening here?