Slitherine

Strange one this.

PBEM, default options, Germany starts Barbarossa July 12 1941, invades, and by the end of the turn has captured Kaunas but no other Baltic States cities..
Allies turn (mine), I retreat but garrison Riga and the remaining Baltic States cities.
German next turn, Finland surrenders.

????

Game turns available.

I guess you mean Finland offers armistice with Russia. This can happen after Finland joins the Axis if both these conditions are met:
1. Less than 6 Axis units are located in Finland
2. Russia control 3 or more cities or rail depots in Estonia or Finland.

Usually Germany will reinforce Finland early in Barbarossa. E. g. with air units redeploying.

How many Axis units did you have in Finland this turn? Can you maybe show with a screenshot?