Blender 2.92 and above - Viewer is fixed :)
Wrong interpretation of emmissive light on alpha in viewport is fixed now. The proper way of appearance is invisible, but still there.
If you check separately R, G, B channels, emissive light (like glare) is there. So when composited over nontransparent its there.
Also when saved this gloving on transparent BG as OpenEXR, can be composited as in previously.
Previous hack for 2.8 is not working anymore in 2.92 version.
Cycles - simply Mix render Alpha pass with Glare and plug into Output node > Alpha socket.
Worked with PNG 8-bit RGBA format. (Doesn't has to be OpenEXR.)
Note: Viewport vs Render F12 issue persist, from devs side -missing transparency of emission in alpha channel is as designed, viewport is broken.