Thanks,

I was searching for 4 tickets on ITA.

So perhaps there were always really only 1 or 2 of the cheaper fares available on those days (instead of 4) but not reflected correctly online until I tried to make the final purchase. That would make sense as after the purchases failed the prices went up on both Delta and Chase.

Just seemed strange that it happened on multiple different departure dates.

Also, it was not only the flights I tried to purchase that increased in price. All similar flights (e.g. the one leaving a few hours later with same connection) also increased in price after the failed purchase.

---------

...just checked on the delta site and the fare class did switch from V to X with the change in price on those flights