Yes as John said, this is less of a bug and more the effects of the new Turf FX. PBR for ground is Parallax Based Rendering. So it has its own height values, aka like real life. Rocks aren't flat, so nether is the PBR Gravel textures.
Do I like this? No, not at all. The devs could of done a much better job and doing a check value here. If track is over X area of a PBR texture, Ignore height value of the PBR. Example: Lay PBR gravel, lay TR19 track. Where the ballast stops on the 19 track, the PBR height kicks back in.
In its current implementation, you must go and change EACH spline point's value to get the ballast from the TRS19 track. 0.13+ height is a good starter, from there its personal preference. I like my ballast raised, like a real RR. So this for me is a PIA. Having to make your customer go in and change each spline points height is absurd. I'd love to see this changed, but I think I'd have better luck in Vegas on this one.
I love and hate PBR. In other titles with PBR its done well, in 19, its a little... 'meh'. The jelly effect as tron said as well is very annoying. Updating old routes is easy as 1, 2, 3. But then it takes a bit of care and love to clean up the underlying problems.
One way to avoid this, but lose a feature is take off Ultra Shader Quality, it disables PBR.