Yeah the revoking issue is indeed related to Viceroyal Duchies (however not owning the Kingdom's de jure capital will as much trigger endless circles of the Viceroy plotting to or simply deciding to revoke the title, only to then again grant it out to some noble per some event or whatever).
I decided to not go for hereditary Dukes wherever I could, as the base concept of the viceroyalties for me is to prevent the formation of large vassal realms by combined inheritages.
For all I found out, "preparing ground", i.e. distributing (viecroyal) duke titles de jure and putting a Viceroy on top, leads to nothing productive, as the AI simply doesn't like it. In Sicily and Croatia respectively, I installed viceroys early on when I got elected emperor, only to then see them struggle for almost a century- particularly Sicily had some severe issue, he kept revoking Palermo, his de jure capital, only to grant it out to nobles again and again constantly- at some point even making a city county out of it until I decided to rectify the situation after inheriting the title back.
Following this experience, I proceeded to set up conquered territory in a way where simple single counts are put under one Exarch holding all the titles and the de jure capital. Consequently, my Exarch of Syria- which I took much later than the de jure territories in Italy and the Dalmatian coast- started to invade further muslim territories right away, while the Croatian guy took almost a century of revoking plots before he eventually moved out to press a de jure claim against the Duke of Balaton in the north.