We're using a banner system to load ads via ajax. The images in the ajax responses are handled by mod_pagespeed, but they're not converted to webp. If I call the ajax url directly in the browser, the image is delivered as webp. Is this some kind of bug or do we have to adjust our configuration?

Example: Visit https://www.whisky.de/forum/ and check the banner on top, then call https://www.whisky.de/?type=9001&tx_sfbanners_pi1%5Baction%5D=getBanners&tx_sfbanners_pi1%5Bcontroller%5D=Banner&tx_sfbanners_pi1%5BcurrentPageUid%5D=1&tx_sfbanners_pi1%5Bhmac%5D=5c7c784ee0e0f4c801dfbb52b15426ceed38550f&tx_sfbanners_pi1%5BstartingPoint%5D=&tx_sfbanners_pi1%5Bcategories%5D=579&tx_sfbanners_pi1%5BdisplayMode%5D=random&cHash=012f2fb0e52bdaaee69016443b9e3f21