MipMapBlur ugly rendering on mac with transparent background

Description of the bug

When using the BloomEffect/SelectiveBloom effect with mipmapBlur: true on windows with a transparent background the effect has a
nice roll-off, especially with higher radius and low smoothing values.

The exact same setup on MacOS renders with ugly banding and what seems like significantly lower mipMap levels, to the point that there is no roll-off.

To Reproduce

I have replicated the exact settings used by <model-viewer> in this sandbox.

https://codesandbox.io/s/bloom-transparent-zpn7iw?file=/src/App.js

Screenshots

Windows, IOS render like this:

MacOS renders like this:

Library versions used

• Three: [e.g. 1.151.3]
• Post Processing: [e.g. 6.30.2]

Desktop

• OS: MacOS
• Browser [Chrome, Firefox]
• Graphics hardware: [M1, Intel HD Graphics]

Additional comments:

I have checked all WebGL compatibility issues I could have thought of, with https://webglreport.com/?v=2.
Both desktops that created the screenshots support OES_texture_float_linear, 'highp' precision. The max samples is different (16 vs 8), but that is not enough to explain the difference since the default mimapblur uses 8 levels and I have not altered it. I can also confirm this does not happen with alpha: false on the WebGLRenderer.

Start with the linked CodeSandbox, especially src/App.js, and reproduce BloomEffect or SelectiveBloom with mipmapBlur enabled and a transparent WebGLRenderer. Compare the MacOS output with the Windows or iOS result while checking the reported alpha, mipmap-level, precision, and texture-filtering conditions. Done means removing the banding and restoring the expected roll-off without breaking the opaque-background behavior.