You can still get the achievement, so the system does in fact keep track... However for some reason it will not show how many you have. But when you get fifty you get the notification and 50 blues. After that who really cares how many you have?

no, it doesn't matter after you get the achievement.

yes, it matters beforehand because:
1) its SUPPOSED to be showing you anyway but obviously isnt (fail update much?)
2) who wants to try and remember every single one you get by writing it down
3) a person likes knowing if they're almost done with such terrible tasks

Hmm it is always undefined, Why?

maybe it's still too complicated to keep track of all the UFOs u've caught? But if it is how can they know when u hit 50? I think it's better to remove that until that "undefined" word turns into a number. It's very funny to see that word ^^.