Hi Brigitte. Thank you for sharing your project. I can see now that you are running up against a limitation in the way the VR Comp Editor generates 3D edits. It's not (technically) that your blending mode is being ignored in the final comp, but rather it's no longer being calculated against the background image as it appears in your edit when you navigate to the edit comp using the VR Comp Editor. This amounts to much the same thing; it's wonky and confusing and I am not excusing so much as explaining what's happening. When you add a 3D edit, the VR Comp Editor script creates a bunch of support comps, and part of that structure is a series of comps that represent the six faces of a cubemap. It's at this stage where your "add" blending mode is, technically, preserved, but is now calculated against zero alpha rather than background pixels.
What might be more helpful to you is suggesting a couple possible workarounds. The easiest workaround would be to add your jpg image as a 2D edit because the precomp structure for a 2D edit does not require generating a cubemap with alpha, and your blending mode survives the journey in a much more straightforward way. I don't see what 3D work you're trying to do with the jpg image, but it might be possible that you could do that work in a separate 3D comp (say, manipulating your 2D element in 3D space), and then add that 3D precomp as a 2D element with a blending mode in the 2D edit.
A second possible workaround that would apply ONLY to the project you shared, and is not universally applicable (or recommended) is to (re) introduce the desired blending mode one level up from your output comp where you'll see your edit precomp is being once again composited against your background precomp. If you were to unlock the edit precomp in order to change its blending mode, your output comp would once again look like your edit comp. The only reason this works in this case, though, is because your 3D edit contains one layer with one blending mode applied. I mention this in case it gets you working again within the project you have, but I don't generally recommend messing around with the VR Comp Editor scripted-and-locked comp structure, because it can be a bit fragile.
I can imagine a third possible workaround that would involve blending the black out of your flame image in order to create a precomp with an alpha channel -- maybe doing a luma stencil blend against a white solid and then track matte the original color detail back in, but this would certainly look somewhat different than doing a simple "add" blend against the background image pixels. The most straightforward way to do this would be as a 2D edit if there's a way to make that work.
Sorry for the wall of words, but I hope there's something in there to help you get back on track.
