I've exactly the same problem with Pixma TS6251 where it sounds like the printer has became B&W only, no colors anymore.
I tried reseating print head, cleaned nozzle more than once via maintenance menu..no way. Sounds like a SW problem as the PGBK prints greatly. Printer identifies cartridges correctly and even replacing them doesn't fix the problem. I saw in some cases FW update fixes but I've already latest. Hard and soft printer reset didn't work. No clue what to do next.