Just my observations but maybe they might be of help to you guys:
I have been busy dialing in CF Nylon filament and had about 20 benchys done without any clogs or heatcreep problems so far.
However when I started printing with PVA Support (or PVA Support interfaces to be exact) I suddenly had some heatcreep problems now and then.
What I have observed is that during long prints (48h+) my flow gradually decreased towards the end of the print. The prints finished but I had some strange behaviour that after a finished print the head was clogged immediately at the start of the next printjob.
Had this a few times, every time the clog reached up into the cold end, about 10mm or more.
I didnt have this problem with the PVA nozzle though, only with the one printing Nylon.
One time I changed the nozzle somewhere between two jobs and saw that inside the hotend something looking like a thin sleeve of plastic at the walls of the heatbreak had formed.
So my assumption is, that the retract at nozzle change might be the source of at least my problems.
The Nylon has the default setting of retracting 16mm at nozzle change, with standard standby temperature. So the filament retracts up into the coldend and starts sticking to the walls forming a sleeve, as long as the print is going, it doesnt fully cool down (standby temp) and still manages to extrude with ever decreasing flow, as the sleeve gets thicker. As soon as the job finishes, the temp goes down to 0° thus completely solidifying the filament and clogging the head.
The PVA has a standard retract at nozzlechange of 2mm as it is too flexible to retract further without forming a knot inside the extrudergears (tested this) and doesnt show this problem.
so my assumption is that the 16mm retract at nozzlechange is part of the problem.
I now got a 52h printjob running with Nylon retract at nozzlechange set to 2mm, hoping that this will solve the problem.
Oozing of the inactive nozzle is present, but managable as it stops as soon as standby temp is reached.
Interesting though. We got an ultimaker s5 at work, which also has a standard retract at nozzlechange of 16mm, but no problems like that. It hasnt been used very much for dual filament jobs though.