This started happening after the update to 1.16. I had 5 golems at my village and one by one they started disappearing until none was left. At first I though they were stuck somewhere or got killed by mobs but I usually find the golems' drops. I figured I'd just wait till they spawn naturally again but they have not.
Resolving as a Duplicate of MCPE-21416.
When a village dweller leaves a village and does not join a new one, it takes 20 minutes for the village to remove that dweller from its list. Iron golems should resume spawning when that timer runs out.