Thanks for trying this out.... I was concerned about this. But I thought MPS would work correctly because we look for accept:image/webp.
We will send a conf file workaround today and also put out a patch soon after.
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SetEnvIf User-Agent CriOS mps_disable_webp
RequestHeader set ModPagespeedFilters -convert_jpeg_to_webp env=mps_disable_webp
I reproduced the problem on an iPhone running the latest Chrome on iOS. Then I put this workaround on modpagespeed.com and the problem is resolved. I also confirmed we do still send webp to desktop Chrome.