I moved over to Generate press and added Webp Express to make sure images were being converted and also the HTML was being changes to <picture>.
On sites where I am not using GeneratePress (currently migrating them over) Webp Express works fine. However, not on the Generate Press sites. All sites are on the same server with the same setup, only difference is the theme.
GeneratePress shouldn’t hinder a plugin for image optimization to work. There’s no code in GP that should cause this. Can you try using a Theme like WordPress theme – TwentySeventeen? Can you also reach out to the support of this plugin and see if they have an insight regarding this?
Can confirm it only seems to be GP that’s this is happening on. I have already posted a query in the support for Webp Express but had no response so I have just changed webp plugins and it’s working now.