Yes, I think you probably put some black in the magenta while you were topping up. Easy enough to do. Been there… I now have a very strict, almost anal, routine I now fill with only one cart out and one ink ready to fill and re-read the labels several times.
Once you had topped up it would still take some time for the OLD ink in the feed tubes to get used up and then you would start to see the difference. To help isolate the “bad” cart you could take a a sample (on a Q tip or similar) and paint it on a paper and then do the same from your supply bottle. Let dry and compare. Once you have emptied the faulty cartridge, cleaned it and refilled with the right ink it too will take a while to replace the ink in the feed line. I do not know how long or how many nozzle checks would be needed. You then might try to print a sheet with nothing but a large block of the subject colour or as close as you could guess to same. Thus you would not be wasting as much of the ink in the other cartridges. Then, I would suggest, printing a picture you have previously printed and compare.
Hope this helps. As i said, I have been there…