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.
Is this still an issue in the latest version? Does MCPE-87453 describe the issue--that is, where patrols do not spawn, are you standing within 32 blocks of the out edge of a village boundary?