First, it appears the time of day setting was incorrect for K1 tank such that it was overlapping with K2. Probably for the last three weeks.
Also realizing that toilet use during softener regeneration is allowing that bit of insoluble iron trough to interact with chlorine tab in toilet reservoir.
Also also, it could just be the softener needs a bit more B2 or RR so increased B2 from 5 to 6 and RR from 10 to 12.
---Time of day settings now correct and softener timing tweaked and keeping a log for use of master bath toilet.