Parallax material breaks on save (4.22)

Title says it all really, my POM material works until the heightmap is saved (either manually or autosave) and then it breaks. No combination of compression settings appears to fix it. I can repair by reimporting the heightmap, but as soon as the file saves it’s once again broken. Standard POM setup, nothing fancy. I’m clearly making a schoolboy error but for the life of me I can’t put my finger on it. Any suggestions greatly appreciated.