Lighthouse not recognising WEBP

Im confused why chrome lighthouse isnt recognising pngs polished to webp.
e.g

[type or paste code here](https://rebuild.mulberrybushtoys.co.uk/images/thumbs/0012055_0010032.png)

See attached images, lighthouse is flagging images but you can see headers suggest all OK.
Ive tried setting browser user agent to light house as I read that this can change how things are served but this didnt change it.
Any ideas?
Thanks