My. my. Quite touchy aren't we? I tried to be courteous and informative calling the app's maintenance staff to an apparent defect. It appears that one delicate ego was bruised by my candor. I guess that makes me a troll. How sad for the respondent who deemed me so.
My thanks to DvDivXXX for pointing out that the app behaviour I observed -- “the fact that mythic items are deliberately left out of the Max Out option” -- is by design and not a bug. In my experience, as I stated in my original post, the "Use" button also remains inactive when I select the "mythic" item. I.e., use of the mythic item seems to be impossible. Hence, my post.
I appreciate the suggestions offered by respondent FinderKeeper. However, I had tried these actions without success before posting. Perhaps the issue is with my browser setteings. I assume that "Also it's quite regrettable to find respectable keenness going faulty on topics of measurable significance and shame, good sir" was intended as an insult, but at least you judged me a "good sir" and not a troll. I appreciate that as well.