Eduard Visser

Update 2. I just captured 3 ominous banner from consecutive raids, which stack together, but not with the already stored ones: the unplaced and broken ones. I am getting the impression that properties, that I am not aware of, the different captured banners somehow differ.

In 1.16.4 I killed a captain, took his omnious banner, attempted it to stack with omnious banners I had captured, placed and broken. I noticed, it did not stack and in the chest gui the list of colors differed. The color list of the captured banner listed no colors, while for the broken omnious banners, the listed colors are: Cyan Lozenge, Light Gray Base, Gray Pale, Light Gray Bordure, Black Fess, Light Gray Per Fess. See attached screenshots.
As soon as I placed and broke the captured banner, the color list was changed from empty to the color list of the other broken banners and stackable. So placing and breaking is an in-game workaround.
Update. I just captured another ominous banner, and noticed it does not stack with the other unplaced banner, nor the broken.

All versions started were 1.15.1 thru 1.16.3, no pre-release. I just checked the logs with bashcode below. I noticed in the launcher that there is a pre-release/latest snapshot.

# cp -a .minecraft/logs ~/mclogscopy
# cd ~/mclogscopy
# gunzip *.gz
# grep Starting *.log|less

I discovered an in-game workaround.
I had already built a scaffold tower on the dark patch, noticed level 112 lit.
Built and demolished a dirt layer at that level.
As soon as I placed some blocks the whole scaffold tower until surface level and a small piece of the patch lit up.
After placing and demolishing a dirt layer approximate size of the dark patch, some shadows remained, but the majority was lit up.
Maybe 3 dirt layers would have sufficed to remove all the sunlight obscuring (air?) blocks.

I added links to video and worldsave.