Played as ethiopia in multiplayer (2 players on one country), bringed troops to italy, occupied italy's almost all provinces including rome (while also fighting germany and the rest of europe) and these provinces started to increase resistance by "scripted value", stacking on each other every 1.5 years (i don't know accurate interval, but war has been long and i stacked 5 "scripted value" and each one adds 20% to resistance).

(by the way, other countries doesn't have that problem, for example, america occupied canada for 6 years, or spain occupied all african colonies)

(picture of stacked "scripted value")
https://cdn.discordapp.com/attachments/319880920523866112/952280247494139914/unknown.png

(picture of resistance situation in america)
https://cdn.discordapp.com/attachments/319880078433583104/952287443883814992/unknown.png