I got the same problem, it was working fine on 5.3.2, well from second time, I set up ocean, coast line etc and few simulations, all was great, but then I changed some settings with thirdPerson actor blueprints and water dissapeared and simulation stopped working. I have created new scene - same, coastline domain does not recognize any landscape I created no matter shape and size.
Reinstalled engine from zero - same, fluid flex simply stopped working at all. Water visible only when its binding box is at the same level with water, I raise it to get flex to detect costline and water dissapears.
Dont know what to do to be honest…it was working so fine before…