If the player has ever been in an area of the map while in Peaceful mode and then switches the game to a harder difficulty, zombies, skeletons, etc. will again spawn in that area; but pillager patrols are not resuming their spawning in those areas, necessitating that the player move out to an entirely new generated region to spawn more pillager patrols.

I have been camped in an area 100 blocks outside a village for at least 20 hours (IRL time), both sleeping during the night and staying awake.  Before I switched the game into peaceful mode to do some building in peace last week (IRL), the game did spawn a pillager patrol while I was in the camp location.  Since switching the game back into a harder difficulty after finishing my build, the other enemies have resumed their spawning; but the pillagers are no where to be seen.  The Wiki says it's a 20% chance every 10 minutes.  I have run this long enough to convince myself this is NOT just bad random luck, but rather a bug - probably related to the notion that pillagers should not respawn in woodland mansions. 

I have also noted that they do not respawn in Outposts if the game has ever been in peaceful mode and the player has wandered near to the Outpost; but brushed that off as an "working as intended" issue.  Now, I believe it is also likely a bug.  I can accept them not respawning in woodland mansions since the player may want to move in after clearing it; but they should respawn in outposts and most certainly continue respawning as patrols even after the area has been moved into peaceful and back out of it.