I can paint across a seam with projection paint but after I "accept" it it changes appearance and in this case I get an unpainted area right at the edge.

I'm guessing this has something to do with the UV mapping but I don't know what.
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here's the model before painting:

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And here it is after:

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Robert...I had something similar to this happen today...using the transparency brush.....BUT I think my mistake was that I was attempting to overlay two levels of the same image....Within the same decal image...

Better explaination...... The UV map has two places with the same patches.... I was attempting to paint over one and have it cover the other,....I was using the transparency brush to smooth out the overlayed edges.....THIS DIDN't work... I must be nuts

Me again....

Robert.....Open up the image in the UV viewer and see if the patch is covered....Looks like a patch edge might be stretched out beyond your image.....From your posted image....that's what it looks like...

I've had this happen at times......HOWEVER....If you're using the "Auto" decal function...it separated everything into blocks that are hard to decifer.

The patch definitely has bitmap under it:

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That is a common problem with most 3d painters going way back to Meshpaint if anyone remembers that program.

I believe it has to do with the interpretation of where the edge is defined on the textures. Higher resolution textures should minimize the problem but only way is to test it. It could be something as simple as where the anti aliasing of the brush is interpreted (inside the edge rather than beyond).

Someday AM will have LSCM and minimal edges...... someday.....