I'm well aware having done it before and avoid it as much as possible.
Do you have evidence that the zones were changed when this diversion was implemented?
According to GO's Twitter Feed, the diversion to Port Credit was already periodically happening in January 2019 when there was heavy traffic congestion.
According to the Metrolinx website, fares were adjusted in April 2019 to lower the fare for trips under 10 km to $3.7.
Q&A about fare changes
www.metrolinx.com
According to the schedules for Milton and Lakeshore West, in January 2019, Port Credit is in zone 11, and Erindale is in zone 12.
According to the schedules for Milton and Lakeshore West, in September 2019, Port Credit is in zone 10, and Erindale is in zone 40.
According to the "fare_attributes.txt" file in the GTFS file for GO Transit, a trip without PRESTO from zone 12 to 11 is $4.40. Whereas a trip from zone 40 to zone 10 is $5.75.
Metrolinx has resources available for developers to build their own mobile applications, using our transit-related information for GO Transit and UP Express.
www.gotransit.com
With PRESTO if you make a trip with a transfer, and both segments are charged the short-distance fare ($3.70), then the total fare gets reduced to $3.70 after transfer credits are applied.
If you use the above example as a proxy and apply it to a trip from zone 11 to zone 12, the fare would be $3.70 using Presto. Cooksville to Clarkson with a transfer is an example:
https://www.gotransit.com/en/plan-y...0&distance=11197&durationKey=PT33M&time=11-00
A trip from Port Credit to Mimico is $3.70 using Presto.
https://www.gotransit.com/en/plan-y...63&distance=9950&durationKey=PT15M&time=11-00
Had the fare zones not been adjusted, it would have been possible to travel from Erindale to Mimico all for $3.70 (a distance of 23 km).