Hey Jacob!
Thanks for the In-Depth explanation on how to fix it! Yesterday I tried a different tesselated material and it’s indeed the same issue there. Though in 4.7 there’s no problem it seems.
I also fixed the issue yesterday by setting the “Max Displacement” in the “master_terrain_blend” material to about “80”. The holes were gone. Don’t know if it affects performance much though or if it’s a good alternative to turning off “Adaptive Tesselation” ?
Also managed to make the foam on the shoreline work! Thanks a bunch! 
Great pack you have here, probably one of the best on the marketplace right now!
And definitely the best river-tool (which was needed baaaadly) until now !!!