The 1400 (and 1410) seems to have been fairly unique among Epson desktop printers in not having the first and last inch micro-banding. I really wish I'd kept mine. If you wanted to avoid it entirely post 1400, then you would have had to go to a Pro printer like the 3880, and as noted elsewhere on my blog, they came with a different set of challenges - maintenance. And now even that isn't an option, as there aren't refillable carts available for the US versions of its replacement the P800 (although Jon reported on LuLa that refillables work in non-US versions).
The workaround I described in that blog post works, and is not as bad as it sounds, unless you're printing in bulk. If you're printing on larger paper like A3 or 13" x19" then you'd typically leave a 1" margin anyway, so there isn't a problem with larger sizes.
This issue is surprisingly common and notoriously hard to diagnose. It does sound like your Ma carts are slowly leaking. Question is why? In such circumstances I'd be suspicious of the little rubber exit valve not sealing properly. It could be faulty, although to have it happen on multiple carts in just one position seems too much of a coincidence. There is a current idea being promoted by forum member LarryB that the remnants of the plastic cover over this valve that you break when you first use a new cart may be interfering with the valve sealing properly. He has posted images of his suggested mod on another forum. If you try this and it works please report back, or even if it doesn't, as we need a body of evidence about this approach.
The other possibility is that variation in manufacturing tolerances for the cartridge bay has meant that the ink nipple in the Ma position may not be as good a fit for refillables as it should be, and you get a slow leak. OEM carts seem more resilient, perhaps because the exit valve is better, or because their complex internal micro-venting prevents them draining. I have no proof of this, but it does seem to explain issues I've had with several printers over the years, plus those of another forum member. At the risk of depressing you further, you may wish to read my lessons I've learnt from using desktop refillables over 9 years post.
P.s. You did mean Magenta and not Light Magenta, didn't you? With K6 curves, you do tend to use a lot of shade 5 in the LM slot, so expect that.