The mangrove blocks such as leaves, woods, planks, slabs, stairs, fences, fence gates, doors, trapdoors, signs, buttons, and pressure plates are sorted at the very last of the creative inventory grouping. However, these wood types are considered overworld wood types, so it needs to be sorted next to the other overworld wood types, conveniently next to dark oak.

It should be sorted based on the following:

Oak - Spruce - Birch - Jungle - Acacia - Dark Oak - Mangrove