The “Bloom” effect in the EEVEE rendering engine looks very impressive but does not visible when rendering on a transparent background.
But this can be fixed.
Bloom effect does not disappear when rendering. But for some reason, it does not visible if the background is transparent. We can check this with putting any background under the rendered image with the “AlphaOver” node.
If it needs to get a bloom effect with a transparent background on output image, we can use the combination of the following nodes:
The result is the image with a transparent background and bloom: