It's a good point that I don't taste it at bottling. But then again I know that my fermentation temperature must've been at least 70F, and probably more, because room temperature averages 65F in my house and the max temperature of the safale yeast I have been using is 68F.
Also, I had two batches ferment in the same room. One batch was in a single carboy and the other was split between two carboys yet the flavor was decided by the batch. Of course they were also both at the same temperature but one was darker which means it would succeed better in hiding off-flavors
I mostly use the same equipment every time. Carboys, stoppers, and airlocks do change though. I soak my hose in sanitizer for a long time and sanitize my bottling equipment two or three times.
Do you use a bottling bucket with a spigot or a bottling wand for all of the batches? If contamination has taken root in plastic, it can be very difficult to get rid of. Does your kettle use a ball valve?