It's possible what you are seeing is more visible than before due to issue #2433 which is:
When images are broken, a blank space is all that is shown. A fallback image for broken images would make this more apparent.
With the 0.19.4 update I notice it a lot more now and see a lot from apnews.com causing this, on my Subscribed view with news@lemmy.world and world@lemmy.world (and others) this morning I count:
Sites that are fine: www.cnbc.com, www.nbcnews.com, www.bbc.com, www.cbsnews.com, newrepublic.com and a bunch of fediverse instances and other "not mainstream" sites from around the internet. it would appear that whatever has changed in code is interacting badly with certain mainstream news sites and the ability to get a thumbnail - I browsed the lemmy-ui commits and a metric ton of 3rd party dependencies were updated, gave up looking for the cause.
I call bull that isn't true at all, yes there are some breakage but somewhere like 3% - 5% and definetly not frickin 50% and if some new release did do that we tend to get quick fixes fast .
On my Subscribed view on lemm.ee sorted by Hot which very recently upgraded lemmy-ui to 0.19.4 (your lemmy.sdf.org instance has not upgraded yet, just checked), of 14x news items/links which should have thumbnails - 8x of them are broken/missing, just over 50%.