Thanks for the additional information, this is all very helpful.
I recommend you get in a routine of agitating ink carts and making a small print, or minimally doing 1-2 regular cleaning cycles every week to help avoid settling of pigment in the carts and internal ink system, and keep the print head moist to prevent drying/clogging. We both use our own products, and carefully track tech support to ensure the best quality inks and carts. We haven't received other reports of Neutral shade 1 (Piezography matte black) clogging or having flow issues, and we have thousands of customers happily using Piezography in a range of Epson printer models, including the newer x890 and x900 series.
If you search online, you will find several reports showing the newer printer models (x890 and x900 series) have a lot of issues, and we have received an alarming amount of messages from people who have only ever used Epson ink (so, it's not ink dependent), but seek our help in unclogging their printers, and it seems like most people are reporting problems with the LC channel.
Epson has designed the newer printer models to not be easily maintained by the user like previous models, and also changed many mechanical parts, which seem to be problematic and burdensome.
From what I have read, the wiper blade in the newer printer models is often the cause of many problems, and is not as easy to keep clean as it is with older printers. It may also be the black capping station (there are separate capping stations for each print head pair), not having a good seal with the print head, allowing air to dry the head when not in use.
I hope this helps. Please keep me posted and let me know if you have further questions or there's anything else I can help you with.
Best regards and happy printing~ Dana