The Design Review will automatically include all the linked artboards necessary for the prototype to work, and exclude those which are not necessary (hence unlinked). You can see which artboards are involved (blue outlined ones), while setting the Share options. The order in which they are eventually presented with the arrows left/right doesn't reflect the way how they are positioned on your Design and Protoype canvas.
Thanks Dan, I ended up deleting the artboard that was giving me issues and was not able to replicate the problem or see if it was greyed out on Share mode.
The main problem I was having was the order of artboards. They were out of order (spatially) but linked in the order I wanted under Prototype. I decided to reorder them spatially to see if that made any difference, and in doing so I noticed one of the boards had a faded back blue arrow, I guess indicating it was leading to two places. I'm only linking artboards to artboards, no interactive elements within the boards, so logically having it go two places doesn't make sense. I unlinked that artboard and relinked it and now the order is fixed in share.
I did not see the "double arrow" before, so I'm not sure if there was an error where it was not displaying or if that's just human error on my part. Anyway moving the artboards around helped me see what was going on.