Refer'd Issue:
embeddedt/aquaacrobatics#16

Minecraft: 1.12.2
Forge: v14.23.5.2854

Mods: 6
[MixinCompat-0.8]
AquaAcrobatics-v1.2.4-1.12.2
GalacticraftCore-1.12.2-4.0.2.281
Galacticraft-Planets-1.12.2-4.0.2.281
jei_1.12.2-4.15.0.291
MicdoodleCore-1.12.2-4.0.2.281

Aqua Acrobatics is a mod that directly ports the movement features that were added in newer MC versions. If this mod is installed with Galacticraft a few odd things can occur.

You are unable to sneak under Half-Slab blocks correctly while holding down the sneak button as the most you'll able to go under is only one, however if you aren't holding sneak after going under one slab... then you can sneak under the rest of the Half-Slab Blocks with issue, besides your eye height being inside the slab.

If wearing the Oxygen Mask from Galacticraft then... no matter what you're unable to sneak under Half-Slab Blocks.

I am unsure if this affects the 'crawling' in that mod as I have yet to crawl through one block gaps.