oh i see it is here:
https://studiobureau.nl/newspaper-infographics-trouw
and its this pic
Screenshot 2020-01-23 at 15.07.07.png
so when an image does not have any alt tag, it will just revert to the usual alt tag that lay theme sets which is the site title.
so maybe what you're seeing in your page source code is just a different image that has no alt tag.
also if you set an alt tag for an image in the media library, you'll still need to re-insert that image into the grid for the alt tag to update, sorry about that :O