This is probably due to auto-import configuration.
Open Tools/Import, choose Configure Auto-Import. Look at the tasks. There is an option called Ignore files preciously removed from library. This option is probably turned off.
If you wish MC to ignore tracks you've manually removed from MC (which is not the same as moving/removing files from disk), you can enable this option. Tracks won't be reimported when they are found on disk by auto-import.
Just realize that this will leave "orphaned" files on disk that won't reappear in MC. If you want files removed, you need to then manually delete them.
