Greyspace phrases (words connected by greyspace characters), function as search terms and are accepted by incategory, hastemplate, and intitle. Only intitle handles them improperly, and only sometimes.

For three-word phrases, it's OK:
intitle:back_door_album (Proper behavior.)
back_door_album (Proof intitle works with greyspace phrases.)
intitle:green_tree_frog (7 pages all proper)
intitle:"green tree frog" (any way you phrase it)
intitle: greenTreeFrog (any way you phrase it)
green_tree_frog (96 pages otherwise)

But for two-word phrases:
intitle:door_back (Three mismatches.)
[intitle:"door back"](https://en.wikipedia.org/w/index.php?search=intitle:%22door back%22) (Zero, as expected.)
intitle:frog_tree (Another mismatch, but again, all words must also be found in the title)
[intitle:"frog tree"](//en.wikipedia.org/w/index.php?search=intitle:%22frog tree%22) (Zero)