That has been so for years and is a well known fraud practice (now that is fraud) of DL - likely so devised so that DL would fare better (i.e. lie) in the award availability surveys, which just look at the results the site spits out and doesn't try to actually book them. When you check back 5 days later, 5 weeks later, even 5 months later, the same non-existent flights will be still there. That is DL knowingly giving you false and made-up award flight options, that are not really there, and not something to do with caching (as it is persistent forever) or anything else along such lines.