Yep I see this on AT&t's post paid network with my Pixel 3A XL as well, one place I really noticed it causing issues is with Facebook and Instagram where Facebook requires constant captions to view any Facebook links I receive and embedded Instagram content in news articles and things of that nature often failed load. It is very annoying.
Tmobile US, VZ, and Sprint all have IPv6, but only AT&T has this behavior afaik.
HTTP proxies are used by some mobile carriers to down-scale media sent thru their radio network to reduce bandwidth. They rationalise that, e.g, a HD video can be down-scaled for a tiny screen with no real loss of fidelity but a signficant reduction in bandwidth. Similar strategies apply to almost all compressible media: mp3, jpegs, etc. More often used outside the US as I recall but sounds like AT&T might be doing something similar. You could try a mobile fetch of a known media file via HTTP and HTTPS then compare them for possible insights (make sure to use a mobile browser to avoid browser-detects). Such proxies are sometimes used for carrier ad-insertion as well so one presumes they detest the widespread switch to HTTPS for at least two reasons. Mark.