320,000,000 limit beyond which the count doesn't go, and when Tech discoveries go about once a turn, ALL of my freights start yielding a uniform 200
Its not the FT rollover. You would get 10 techs in one turn, not 1 per turn, even if stuck at a rollover FT low science rate, it would rise hugely in the next turn, not stay at 200. The total population is not relevant at all in the direct bonus calculation.
An abrupt change can be caused in an Oedo year by a change in Key Civ matchup, or advances possessed in the Key Civ. With 320M, you are supreme, and the Key Civ is the purple. However even at parity (at diety level), the key civ will trigger about a 1900 science per turn around the end of the tech tree, which is about a 1200 cap, not 200.
Assuming you are in Democracy, delivering across a large map between cities with lots of trade (size 20+ and 50+ base trade arrows), and proper improvements (SH in source, and matching airports), you can't get this low even with undemanded goods. However, if you are delivering on local roads to nearby villages of your own, this is possible. You could have been getting 400, and drop to 200, if an enemy has moved and sits on the return path.
Attach the .SAV file.