photo burst "make a selection" does not work - can't choose the best image

Photos on my Mac is behaving badly. Hopefully I'm just missing something here. (BTW - I know this post is long, but I didn't want to waste anyone's time suggesting things I'd already tried...) (I'm on a MacBook Pro, High Sierra)

I've just imported a few photos from my iPhone (and deleted them from my iPhone after import).

One imported photo is a burst of 5 photos. The default thumbnail is blurry (image #2 of 5).

I selected the photo & clicked "Make a selection". The first of the 5 images is ok, the other 4 are blurry. On that one good image, I clicked the little circle at the bottom right, & it turned into a happy little blue check. I made sure the others were not selected - no blue checks. So far so good.

Here's the problem: When I click "Done", the thumbnail image defaults back to the 2nd image (blurry), keeping all 5 photos in the burst. I cannot delete the bad photos.

I tried it again. Then I closed the app, reopened it, & tried again. I searched Google & read a couple of related entries here on the forum. One (https://discussions.apple.com/thread/7939693) suggested to "Click Keep Only Selection." However, that option was not visible.

Also odd: When I select the one good photo, the blue check is there, but the thumbnail beneath the photo is greyed out. The de-selected photos' thumbnails are not greyed out (scroll down for a screenshot of this). That didn't look right. I double-checked the other support entry - yep, it says the blue-checked image is the one to keep.

Just to double-check this, I de-selected the good image, and selected (blue-checked) the 4 bad images. When I clicked "Done", I finally got the "Keep only selection?" option. I click yes, and it kept the 4 bad, discarded the good. Yay for UNDO.

So it's possible for it to work properly - it let me discard 1 and keep 4. It just won't let me select the 1st image and discard the other 4. Since the other 4 are much blurrier (and tilted), I know the 1st image isn't a merged image.

Next I tested it by trying to keep 2 images, the good one + 1 blurry one. I clicked 'done' and there was no "keep only selection" option - it just deleted some of the photos. Worse, it seems to have merged the photos - the good one is just gone. Yay again for UNDO.

Also, I can't get to the good photo to perform simple, basic edits. When I back out of "Make a Selection" then click "Edit", the good image is not visible (it's merging the images, right?).

So - is this a bona fide bug or am I missing something?

(And Yes, I know that even the "good" photo is not good - I want to get this right before working on good photo bursts.) Thanks in advance for input.