Did you try a different output format? Just curious if it's a PNG only issue
bloom generally fills in everything. there is no alpha that i know of with bloom. you need to make that a post effect if you need an alpha. you can also use multiple layers in compositing. one with an alpha. one without.
Can you elaborate on the comping process? I ended up adding the bloom in post separately in adobe, but would love to know what kind of pass I need to get the bloom as a separate aov. Or is beauty pass the only option? In the instance that I asked this question I specifically needed a png sequence, but would like to know more in general about how the lens effects are exported