I don't know if this is a known issue, or if anyone knows of a workaround, but when I add a Decal to a part & turn on the option to "Use Underlying Appearance" on the "Illumination" tab the whole surface becomes invisible as below.
Is the underlying material transparent? using that checkbox will use the illumination settings (including transparency) of the material it's applied on top of.
The underlying material does have transparency on it but not very much & this has never been an issue in the past. I also changed the material to Aluminium & had the same result of the decal vanishing.
The image below shows the old issue flexible saved in SW2017 that I can open, edit the decal & the "Use Underlying Appearance" box is already checked but the decal is visible. I can uncheck & check the box & the decal doesn't vanish.
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