We've been discovering the mod's cool features at random but after having hit level 9 we tried meditating on the moon but the progress bar just appears for a quarter of a second and the disappears without filling. I'm not sure if the custom sky is the issue or what but I can't find a way to get past this point.

Same here.
Minecraft-Version: 1.12.2
Forge: 1.12.2-14.23.4.2760
Monk: 1.4
Modpack: Direwolf20 2.3.0

Okay, I've found a workaround: You have to sit as second person in a boat (ask a friend to sit in the boat and then rightclick it so you sit behind him). Now stare at the moon and it will work :)

I'm also experiencing this on a server:
MC 1.12.2
Forge-14.23.5.2772
Monk 1.4

No ability to try the two-person workaround. Has anyone figured out the cause?

I've made a bugfix. It's not the best way but it will work.

if you want to you can skip this level by pretty much using a command /monk setlevel 10 and it should skip the entire level displaying that youre not monk level 10, sadly i think the water walking is quite glitched on this pack okay i just needed to reload the save to make it work, remember to quit to menu and load up again to make it work properly