Automatic rebooking got me from xxx-VIE-AMS in the morning, the xxx-VIE leg being cancelled, to xxx-VIE-FRA in the afternoon, overnight, FRA-AMS next morning. I suppose that late VIE-AMS might have been full?

The bot was good with providing me plenty of options for rebooking from xxx-WAW-FRA, xxx-VIE-MUC-FRA, even a mix of LO/LH in xxx-WAW-POZ-FRA, same xxx-VIE combined to a different VIE-FRA....
You'll notice I was hoping to get to AMS not to FRA, but bot was not smart enough to recognize that. I think it saw xxx-VIE-FRA on the day, an irrops flag, and tried to offer me all xxx-FRA options it could find.

I had to call in and insist strongly to be booked on xxx-WAW-AMS in the morning, which had plenty availability and was the most logical (albeit OAL) rebooking. The rebooking was not ticketed properly and I had to fix it in the morning at the airport but that's a different grump.