Update from GoldenHelmet:
The reproduction of this bug is equivalent to the reproduction of MCPE-141499 and MCPE-23416.

1. Player comes within render distance + 2 chunks of the structure, but not within simulation distance. This triggers the chunk to generate because it contains tile entities.
2. Player moves away so that the chunk is no longer within render distance + 2 chunks.
3. During autosaving, the player comes within simulation distance of the chunk.

I am not 100% sure that step (2) is necessary, but I think it is. My theory is that because the chunk was outside of render distance in step (2), it only loads block data when it comes back into render distance in step (3), and then gets ticked before it can load entity data because the game is busy saving other chunks.

The following more precise steps work in 1.17.32 without the Caves & Cliffs experimental generation. In future versions or with the experimental toggle enabled, you will need to use a different seed and location.

Steps to reproduce

1. Set render distance to 8 and turn on the autosave icon in video settings.
2. Create a world with seed 1212557532, simulation distance 6, and coordinates enabled.
3. After you spawn, /tp 800 ~ ~, make a nether portal, and enter the nether.
4. In the nether, /tp 95 ~ 216
5. Fly east to X = 115 and flying around the area, never going to X < 115 until the auto-save icon appears.
6. When the auto-save icon comes on (showing that autosave is happening), due west (-X), staying near Z = 216, until you get to X = -60.
7. /tp -60 78 216.

Expected result

There is a blaze spawner.

Observed result

No blaze spawner.

I came across two rooms that should've generated blaze spawners, but they weren't there. That was the closest one to spawn (-80 ~ 192). The second fortress (736 ~ -320) luckily had a single spawner room with a spawner. The coordinates of each of the spawners is in the images, and here's the seed; 1212557532. Also sorry about the poor image quality.