looks to me like an air leak at the front of the cap, also, i'd check to make sure that head is still installed nice and solid. it might be coming loose at the front and the cap springs pressure is being absorbed. also, the cap springs maybe getting weak. do you have your carriage set to low when you are doing your diagnosis. make sure you are getting maximum contact on your head. it's most likely not the supply side of your head because it is the bottom of both sides of the print head. to completely eliminate a supply side possibility, put the cyan damper/lines on your magenta manifold and do a test. most likely your cyan will also fall out. if not and the cyan prints perfectly you have a supply issue. but i doubt that will happen.
couple things to do.
1) make sure your cap is making good centered contact with that head. paint the outside rim of your cap with some black ink. gently park the head on to the capping station so that the magenta head imprints the ink on your cap onto the bottom of your head. get a flash light and see if the imprint is centered.
2) get the print room very quiet. pull a vacuum with a syringe from the pump or waste line. listen for an air leak. the best way to do this is with an automotive stethoscope. they are very cheap and can zoom in on the exact spot of leaking air. if you hear an air leak do the following.
3) press down on all your caps. see that they are all behaving the same way. your magenta cap may be stuck at an angle forwards.
4) see if the magenta head's cap spring seem strong. if it seems weak (it could also have fallen off), take apart your capping station and stretch out your cap springs on the magenta head.
5) pour solution into the magenta cap. does it drain quickly? if not there may be a clog underneath that portion of the head.
6) if you have the kind of cap tops that are removable. switch the magenta cap top with a functioning head. see if the issue follows the cap top or stays at the head.
7) i doubt this is the problem but switch pump lines with the magenta pump line so a functioning head's pump line is connected to the magenat head's drain line. does the problem go away. if so, your pump is not getting enough vacuum to the head. also, manually pulling a vacuum from the waste line and doing a test file can be a good diagnostic test.
but i would first look for an air leak at the front of the magenta cap. also a cracked manifold can greatly reduce over all negative pressure inside the cap. to see if there is a small fracture. place a damper with a syringe attached to it on the magenta manifold nipples. put some fluid pressure on the manifold (both front and back nipples). does the "moat" around the nipple fill slowly with fluid or does it remain dry (you have to remove any protective head covers to see this).