I'd suspect a bad SD card. First step - put it into a card reader, connect it to your computer and run
PhotoRec - CGSecurity. If the pictures are still on the card, even if they're deleted or lost, PhotoRec will find them (it will find any of many file types, there's a list on the site, regardless of whether they're listed, what file system they're on, etc. - it just looks for areas on the card that look like jpgs, gifs, docs, etc.) If it can't recover the files, they no longer exist. (
DON'T do anything with the card but remove it from the phone until you run PhotoRec on it. Anything you do to it in the phone could overwrite one or more of the pictures.)
It's a free app, but if it recovers pictures like those, you'd be willing to pay for it.
Next, while the card is still connected to the computer, run
h2testw on it (back everything on the card up to your computer first).
If it's a SanDisk card, contact them - they're very good about replacing bad cards. If it's a Samsung card, they may be almost as easy to work with (I've never tried, because I've never bought their cards - SanDisk is the other manufacturer [there are only 2] and SanDisk charges less for the same size and speed card).
Anyone else? You take your chances. They're buying whatever chips they can and putting them together into a card. They may honor their warranty, they may blame you for damaging the card, who knows? It may even be a counterfeit card. If the card is, say, 64GB, and h2testw reports a 2GB card, believe h2testw. (Once you try to save more than the real size of the card, files get deleted.)
I've taken a load of pictures with my Note 3, using a few different camera apps, and a few custom ROMs, and never had that problem. But I'm using a SanDisk card.