Hi, did you check the weightmaps? So you could try to fill any of the weight maps (in landscape mode, “right click → fill layer” I think, cause if they are all empty you should get a black landscape). You could also try to export the weight maps for the three layers (from the landscape mode) and check that they are not all black.
Also if this is a material that you have used before 5.3, maybe it got corrupted in the update (in 5.3 all the existing landscape layer blend nodes from previous engine versions seem to have been corrupted, so maybe 5.4 is also not backwards compatible to before 5.3). Then you would have to recreate all the weight maps, and also rename the layers.