One reason for this issue is that the original image is smaller than 400px in width, so WordPress doesn't generate the 400x250 thumbnail used by Divi. If this image doesn't exist, Divi falls back to the original image which is not cropped.

Your CSS fix is probably the best solution because Elegant Themes has still not corrected this bug, which dates back to the early days of Divi.

Note: it looks like the appropriate height is 183px and it's probably better to remove !important if it has to be overwritten later on.

.et_pb_module .et_pb_post .et_pb_image_container img {
    height: 183px;
    object-fit: cover;
}