I think they are several parameters at play, materials and process:
Materials: I used the Vallejo PU Gloss varnish as a 1st coat (no issue there) then the AK Ultra matt. Maybe a material incompatibility exacerbated by the processing issues.
Process: I used an airbrush. As the gloss varnish is very thick, I thinned it 50/50 with airbrush thinner. And, as I was told was OK, I applied the AK varnish about 40 minutes later (I was told 20 minutes is plenty enough), unthinned as it is quite fluid.
The paints themselves had plenty of time to dry. At least 48 hours and for that cape it was more than 1 week.
It seems I should have let the gloss varnish to dry at least 24 hours before applying the AK. Trouble is that I used the same process gloss then matt about 30 minuites later without problem but it was both Vallejo PU varnishes.
Anyway, the cape has been striped, re-primed, and re-base coated. I will do the highlights and shadows tomorrow. Maybe I will have time to do the touch-up to the areas affected when I stripped before end of month.