Unlike for normal battle, the current patch does not scale the base chance up by multiplying to by 2 -> to make it 100 on the typical case where both sides have 50% chance of winning. 100 is the value that one gets when combat_odds_scaled_veterancy is disabled. This needs to be fixed.
New version that has the scaling correctly.
When implementing combat_odds_scaled_veterancy ( https://www.hostedredmine.com/issues/804347 ) for regular combat, passing 100% chance as a parameter from any diplomatic battle seemed quite wrong. If challenge affects veterancy odds in normal combat, so should it in diplomatic one.
As part of #43200, this has also been posted to http://forum.freeciv.org/f/viewtopic.php?f=14&t=92051