Having this happen regularly now
For the last 2-3 flights I've taken, I've had a segment random drop off / come back on the page, here and there. The first one I resubmitted was immediately credited, which was great, until two days later, I re-received my original flight that had been there for two weeks... so it was off to de-dupe.
It's interesting, it seems to happen RIGHT AROUND the time I have new mileage about to credit, so I'm going to wait this one out and see what happens, now that I have two segments this week, plus the one that vanished from last week, to come.
Additionally, after months of no real issues with logging in and such, I'm regularly getting those varied site errors saying that pages don't exist, we can't complete your activity, and so on. Ugh. At least the mobile site has been working fairly well for me on my iPhone.