Are you dragging in all 6 images at the same time? If you drag the same 6 images onto a blank area of the pasteboard, do all 6 import correctly?
I tried dragging all 6 in at once but only 4 showed up. So then I tried replacing the 5th one (which was a duplicate of the first) with what was supposed to be the 5th image, but when I did that not only did the 5th one get swapped out for the current one but the 1st one also got replaced with the 5th image.
Hmm, "I tried dragging all 6 in at once but only 4 showed up" is definitely not what's supposed to happen. All the images should show up in the repeating sequence, even if the Repeat Grid wasn't originally big enough to see all of them. Does this happen with any 6 image files on your computer, or just a specific set of 6? Does this happen if you make a new, blank XD document and make a simple Repeat Grid from a plain rectangle -- or does it only happen with a particular XD document?
The second part is expected, though: when you drag in a single image and a repeating image sequence is already in use, it will replace the image you dropped on top of in each repeat of the sequence. Please upvote Change image only in single repeat grid item – Adobe XD UserVoice if you'd like that to change.
Another thing to try is to see what happens if you bring the right edge of the repeat grid in a little bit. Sometimes what happens is that you get a sliver of the next column, which populates it. Let us know if that helps!