The description of the "Minimum" quality parameter in the configuration form for the PngQuant processor says:
If conversion results in quality below the min quality the 24-bit original will be output.
But in that case a file with 0 bytes will be output instead.
The command line help of pngquant says:
--quality min-max don't save below min, use fewer colors below max (0-100)
IIUC this means that there will be no output at all if the quality is too low.
Note: As a workaround until this is fixed one can set the minimum parameter to a very low value to this case is never hit.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Thanks for reporting this issue and suggesting a workaround.
I assume it still affects the latest version of 8.x-1.x so updating the version to that. If it's been fixed since please leave a comment about that.