That is because tinted glass blocks light much like a regular block would if placed there. The reason that the block underneath is completely dark is because that block is getting a light level of zero. Hope this explanation makes sense.
I have attached vanilla screenshot of the issue
The first screen shot is from 1.14.4 and the second one is the same world in 1.16.1
Upon some investigation and experimentation, I remembered that I had at one point updated this world to 1.18.2 and then immediately backtracked this update. Going back to a backup I made before reverting, I have found that the structures mentioned beforehand are unaffected. I am therefore lead to believe that this problem I reported is my fault and should be resolved.