Found the problem I had around “old image” being included in the notifications. I suspected it to be a cache problem, and indeed it is. I am using Cloudflare for my domain and Cloudflared tunnel and it turns out Cloudflare was caching the image and because the names of the images are the same (e.g. _0.jpg) I always got a cached (old) version of the image.
I solved it by introducing a cache rule to not cache “jpg” images. As described here: Home Assistant sends cached images in iOS notification? - #8 by msl2424