Had the same issue. In my material I used a depth fade on the opacity which caused the decal to disappear when getting too close. I wanted to use the depthfade as a mask but sadly it also affects when the decal shows when getting too close to it.
Just wanted to answer this question in case anyone else ever has this issue.
Yes, this was exactly the issue for me. Depth fade in the material casues decals to disappear when close even if you set the fade distance to 0. So just removing it completely fixes the issue.