Hi
I just noticed that the invisible part of my PNG backdrop (the alpha part) is showing up as black in reflections when using High or Ultra Render Quality.
thanks for your report. We are already aware of the issue and have implemented a fix, which will be released soon. See also this existing thread:
RE: mirror not rendering alpha cutout correctly (3.4)
As described there, a possible workaround would be to set the Material Type to Foliage instead.
Hope that helps!