FWIW, I had the experience, in my mid 20's, of having my polar reality abruptly move 90 degrees off north - over the short span of a few days. Hard to describe briefly, but the effects were profound and of long duration: I could still adequately deal with technical situations but suddenly had considerable difficulty in more subjective ones. Developed a tremor on one side and some other physical symptoms, became dyslexic... overall it brought a bunch of unexpected and unfamiliar things that netted out to a substantial loss of competence and cogency. It almost certainly came as an infection from a household pet that had been acting strangely. Either an ear or brain infection, the docs thought the former and I thought the latter, but they found neither and so just treated the symptoms. Time (6 months+) eventually made it mostly go away, but I remain somewhat lysdexic ever since.
Point of this is not to solicit sympathy (or criticism) but to share my intensely personal experience that the behavior of a seemingly healthy, rational, young person can suddenly and profoundly change due to obscure, invisible circumstances. The distance between normal and somewhat bonkers is, at best, paper thin. A person thus affected most likely will recognize something is off, but will compensate and try to conceal the effects.
My suggestion is that crew members should not be reluctant to surface their concerns about 'off' behavior by others, and managements should respond to such in a measured way by using standardized psychometric testing normally and then repeating tests 'randomly' on such occasions. That, plus a little tea-leaf reading, should be able to separate the merely weird from the ill and disabled without making too many waves in the operational envelope.