I’ve been having the same issue for the past week or so it seems your workaround of uploading straight into the gallery UI instead of using images from your media library works. I did not reinstall anything and did not start a new post. Just clicking that “upload” button instead of the “Media library” button was enough for it to work in my case.
@nikeo identified the issue and suggested I disable the option “defer loading offscreen images” found in the appearance => customize interface under nimble builder > site-wide options.
This helped solve my issue of images not loading.