Revised description by Auldrick:

Several years ago, it used to be that crops (wheat, carrots, potatoes, and beetroots) could not be planted in blocks at light levels below 8, and if a planted crop's light level fell below 8 it would quickly break and drop the original item when a block update occurred in an adjacent block. This no longer occurs in 1.21.0 (actually, since much earlier). Crops can now be planted in any light level, even 0, and block updates never cause them to break. This ruins several designs for crop farms.

Steps to reproduce:

1. Build an enclosure with an interior size of at least 5 x 5, made of opaque blocks. Don't include any light source inside it.
2. Embed a water block in the center of the floor. Replace some of the nearby floor blocks with farmland.
3. Try to plant crops on the farmland.

Expected results:
You can't plant crops because the light level is 0.

Observed results:
You plant the crops.

Next steps:

1. Run the command "/randomtickspeed 500".
2. Place any solid block on the floor next to a crop.
3. Place any solid block above a crop.
4. Place a wooden pressure plate on the floor next to a crop, then step on it.

Expected results:
Each step causes the crop to break and drop an item.

Observed results:
The crops don't break. No matter how long you wait, they just sit there.

Additional information:
In Java, crops can be planted at light level 8 or greater and break at light levels below 8. The Bedrock behavior could be described the same way but with light level 0 instead of 8. That is, crops can be planted at light level 0 or greater (i.e. can always be planted) and break at light levels below 0 (i.e. never break because no such light level ever occurs). So this might be as simple as an incorrect value of a parameter or constant.
 

Original description:

Whilst building a potato-farm in survival I noticed that the crop in the middle - which is supposed to pop off on any block-update as the light-level is too low - didn't break. No matter how low the light-level is, the crop still stays on its place and can even be bonemealed. Light-updates don't seem to trigger it, nor does the blockupdate from the adjacent pressure plate. When testing in creative and waiting a little the crops díd pop, but incredible slow. It therefore seems to me that the crop only realizes it's too dark when it grows a stage.

This bug makes constructing a villager farm using this crop-in-low-light behavior pretty useless, so I hope it gets fixed soon.