That's weird as definitely text. I can highlight and edit those lines and change font etc. I did have a similar problem on one of the pages with it saving text as an image. Had to save text to word and repaste.
The only option I can find that would do this is the Create bitmap copy available on right click of the heading text.
In my version of Web Designer (19) it only converts to png image format, whereas you are getting a webp image.
However as you can change the font style this rules out that option as it is not editable once converted.
. . . . Had to save text to word and repaste . . . .
I assume you have tried this with the home page text by creating an empty text object and then pasting the text in?
To isolate where the problem is, does the conversion occur if you start a new project/website and add 1 heading in h1 style and preview it - you should be able to select the text in the browser if it has not been converted
Have had to delete the text strap line. Insert normal text and save. Then update text to H1. All working but because title is on 2 lines it's now saying too many H1 tags as reading each line as H1.
. . . . because title is on 2 lines it's now saying too many H1 tags as reading each line as H1 . . . .
That is normal for several website creating programs.
There is a 'messy' work around you can do, however IMHO it is not worth it.
SEO checkers are advisory devices detailing what they have found and giving recommendations, which can be implemented or ignored.
Some items such as 'keyword stuffing' should be fixed, however items such as formatting should not be taken literally as 'this is wrong - fix it!'.
Having, for example, 6 more h1 tags than is theoretically necessary is not going to break the site or reduce its rankings.
Search engines use 'intelligent' web crawlers to go through websites, in much the same way as the SEO checkers, and are aware of many of the techniques, eg 'keyword stuffing', to try to increase the rankings, it actually has the opposite effect, and they ignore formatting tags such as <h1>, <b> <i>, and more.