When using BetterBeds without other rendering mods such as Sodium this mod breaks the smooth lighting of beds. This was tested in Vanilla and the issue was not reproducible and the same for Sodium where without this mod it works but once you add BetterBeds the smooth lighting for the bed render doesn't work properly. It looks like this is because it does not use the same value for both parts block entity making it look broken.

In the above image the light level of the right block was twelve and the left block was eleven. (Block light only, sky light was zero) When tested in vanilla it appears the BlockEntityRender prefers to use the higher of the two light level values. Meaning this could be fixed by checking which bed section has the higher light level and then passing the value into the render. Below is what the same image looks like without BetterBeds.

This is because in the models they wrote "ambientocclusion": false so if you want smooth Lightning you have to remove this line (I'm gonna make separates packs and a pr so if the pr don't get merge you can at least use the RPs to override the mod).
(I won't include myself and leave all credits to motschen as he's the one that made the models. I will only pack them and just remove one line in like 3 files or so. So well, it's as i did nothing.)

PS : And btw the fancy beds RP I'm gonna make will be compatible with enhanced block entities which is a mod that does the same things and more.

PS 2 : if you don't use fancy or connected beds enhanced block entities has an option to switch smooth lightning on and off on beds

Here's the RPs.
I don't have the time to make the PR so i will do it tommorow
Connected Beds (with smooth lightning).zip
Fancy Beds (with smooth lightning).zip
Better Beds (with smooth lightning).zip
You don't have to use the base betterbeds RP to use the fancy or connected. Use only one.

i've done #6 to add this in mod

Fixed in 1.3.0 (thanks Julien)