Thanks for the background. Very helpful. Next time I'll wait until I don't need a VPN connection.

Another strange issue is that when logging in with my M&M account (U.S. based), defaults reset in address and phone fields - but not for the U.S. but Germany. This has not happened when going through the booking process without logging in.

The comment on pricing/seats of course makes sense. But the issue was actually simple - enough seats (3) were available in the respective fare bucket and I was able to book these eventually after the website showed much higher pricing when resetting the number of passengers from 1 to 3. The solution was to log out, log in again, start with 3 passengers, and voila proper pricing appeared again.

In terms of dynamic pricing - I always run pricing first in ITA Matrix and pull the fare codes. This has allowed me to call certain airlines (e.g., United) and have an agent pull routing and code in short order and book a ticket. I have noticed a few attempts at dynamic pricing and if needed have booked via Expedia. The three airlines which in my experience do not always show all ("cheaper") fare buckets are LH, LX, and AC. In some instances LH however has shown cheaper fares that I was able to see otherwise.