So what's your question? How to prevent it? How to fix it?
I've found that WMP is poor at managing metadata. After i've ripped a CD and WMP can't find the corresponding matadata i use TagScanner to find/replace the info. (make sure that the ripped/downloaded music folder isn't added straight to WMP library and temporarily stored in desktop for example)
Then once you're satisfied with the metadata add the folder to the WMP library. Everything seems to work fine as long as WMP doesn't have to do to much work LOL!