Hi @ColeMan, welcome to the community.
This isn’t an issue I’ve seen before. My first thought is that there may be some issue with the parent tile. Are you able to share your tileset files with me so I can take a look? You can DM them to me directly if you don’t want to post the files here.
Can you test the imagery and the terrain separately? You can apply a locally-hosted TMS raster overlay to a tileset like Cesium World Terrain. If the imagery disappears when forbid holes is checked on Cesium World Terrain, that will be helpful to know.
Also, you can try testing just the terrain by assigning a basic Unreal material to the tileset. Let me know if the whole tileset disappears when forbid holes is checked even if you apply a basic material.