Please fix those annoying 1.8 light gliches:

Not sure if that's really related to this issue but may help
I remember getting this issue (or an issue similar to this, I'm not sure) when was testing my mod on 1.16.1 or 1.16.2, I checked the lighting in the F3 menu and only the (block?) lighting on the client was at 0, server lighting seemed to be right
I thought it was a Sodium bug (I was testing with Sodium) or a bug with my structure generation so not sure if that's what happens here
Edit: it may actually have been even 1.15.2, this was like almost a year to half a year ago so I don't remember it exactly

Lighting in 1.16 is mostly done on the server and then sent to the client, so my ability to fix this may be limited. Some of the planned enhancements to lighting may fix it as a side effect, tho. I also don't have a setup to reliably reproduce this.

The currently known way is to normally set smooth lighting to maxmium and then set "Light Smoothing" to yes via canvas settings although i am not sure if this will be the best workaround.

to reproduce you can install any mod that adds blocks than place those modded blocks and remove the mod. Modded blocks will be removed but lightning won't be updated until you place a torch and it will look exactly like this