This is occurring in dictionary, quizzes, and new word lessons. Not sure if it is affecting others, or due to my network setup.
Have seen the issue occasionally before for specific words. You should be able to add a listener to the image elements and hide them if the image fails to load, instead of showing broken images, to reduce the chance of a broken experience.
Not sure about 24 hours ago, but this reset one was due to a config change on my media server - moving to a much faster setup (caching wasn't really working right on the images, and I'm rolling out the new one in steps)
If you look it up in the dictionary, does it say unavailable? It seems to me that on occasion, it will not show up the first time it's converted to webp (not for you, but for *anyone*).