Pictured here is every Cake in my modpack, with Magma Cake on the right. They can all have candles placed on them, apart from Magma Cake. According to the developer of Amendments, cakes can have candles placed on them if they extend CakeBlock, which yours probably does, as it can be stacked into double cakes (another Amendments feature). However, something that My Nether's Delight has done must have broken the candle behaviour.

amendments-1.20-1.2.19
MyNethersDelight-1.20.1-0.1.7.5

Minecraft 1.20.1
Forge 47.4.0

MagmaCakeBlock does not extends CakeBlock, the stacking feature is provided by MND itself, and Candle Cakes has to be registered and handled by the mod that owns the Cake.

Unlike normal cakes, Magma Cake has facing, for supporting Magma Cake with Candles, a proper class should be made instead of vanilla CandleCakeBlock.

This issue is still valid since Magma Cake should have candle variants to align with vanilla, but it has nothing to with Amendments.