This bug already happened in 24w33a, which is the first snapshot of 1.21.2
The test video link: https://drive.google.com/file/d/1I9-mFobl9Pu91OG0SrjDP9uSLbbDXEv3/view?usp=sharing
(I am not good at English and I am sorry for that)
Here is the link of the video of running this iron golem farm in vanilla environment (Includes 1.19.2, 1.21.1, 1.21.3 and 1.21.4)
The link: https://drive.google.com/file/d/14Mau3iC1b0nvJ6KQ08BXDWw5VCstz7sn/view?usp=sharing
However, I found out this bug already happened in 1.21.3.
In the video, there is no problem in 1.19.2 and 1.21.1.
In 1.21.3 and 1.21.4, I respawn all the villagers to see if it is MC-277889. Unfortunately, it isn't.
My thought
(There is a redstone clock, which is about 35.2s)
I think it is because the cooldown time of spawning iron golems is abnormal. Sometimes, when I run the clock one more time, the villagers can spawn 4 iron golems at the same time. Therefore, I guess there are some problems of the cooldown time of spawning iron golems.
@Jiingy The new map is a vanilla environment iron farm, it is the same to the previous one. The bug is still happening
It also happened in vanilla game, no mod cause this