If a (specific) minecart is on a detector rail, it does not output a signal when it should. If the comparator is updated (right clicked), the correct signal will be displayed. I have attatched screenshots demonstrating this, using a valid command (/say) and then an invalid (/sa)

NOTE: Where I encountered the bug was while using a command block minecart in the nether, i have not tested it any further than this. I am assuming it does the same for hopper minecarts and chest, and in the overworked, but I may be wrong.