Epson EcoTank ET-2720 Weird Head Cleaning Results?

My understanding is that when I get horizontal white lines cutting thru color or black printing, it's usually a clogged printhead. My customer had this issue, and nozzle check seemed to confirm it (lots of breaks & fractures in the 4 colors). (All his ink tanks were 3/4 full.) I ran a total of nine head cleanings (& subsequent nozzle checks). After head cleaning #1, black got a little worse. After head cleaning #2, black DISAPPEARED (no blank ink at all), leaving Cyan good, Yellow good, and Red looking patchy. Then Red faded out after cleaning #3, and Yellow faded out after #7. (The lines broke up and fully disappeared.) But the weirdest thing is: BLACK went from 0% showing up in #3, to 100% saturation in #4, just one cleaning. It was like a switch was flipped. (FYI I tried a firmware update and factory reset between #7 and #8, no improvement for Yellow & Red.) ---- (1) What would make all four colors behave as if the head cleanings are depleting the ink, not restoring flow? (2) What would make BLACK suddenly re-appear completely after one cleaning? As if a software switch had been flipped?