3 things could be happening:
1) the movie's release date in your region may be added or chaged. (your region is specifiable in the preferences)
2) TVR does not consider a movie missing if it is not up to the relase date of the movie. If the movie has passed its reported release date then it may suddenly 'appear' in your missing list (in reality it was always missing, but previously it was suppressed) - again there is a preference seting for this
3) The release date changes consierably in TMDB and the year has been changed. So for example if 'Moviename' has had its release date changed from 1/1/2001 to 31/12/2000. This could cause 'Moviename (2000)' to be seen as missing as it can only find 'Moviename (2001)'
Does that help?