Unable to place woodland mansion, end city and monument in a flat world + another weird bug

When trying to place a woodland mansion, end city or a monument in a flat world, the output says "Failed to place structure", and does not place the structure at all.
Plus I have one more issue to report. When generating a village using the place command in a flat world (can be any village type), some weird black or faded block appears. Any mob (including player) cannot come to the block and will have some weird shaking. Going to the top will cause the mob (or player) to shake and float. But it is possible to get out of the block by walking backwards. I attached a picture about this weird block.
Plus, I want to thank you guys for creating this feature. I no longer have to search for a village, I can just create it!