For all of the lighting blocks (such as glowstone, shroomlight, froglight etc), the lighting intensity of different sides are not the same. For example, the top side is always significantly brighter than the lower side.
MC-269524
Lighting Glitch
Resolved
Duplicate
0
Rudy Zhang
(Unassigned)
Unconfirmed
Lighting
1.20.4
Retrieved
This looks to duplicate MC-54479 based off of similarly resolved issues.