I just did a few tests on the tutorial Sentry Bot.
So far it seems that either enemy always acts first or there indeed is some kind of underlying default turn order.
I was thinking that maybe the character with higher base Initiative would act first (sounds pretty logical and fair) but sadly it's not the case. Sentry Bot has base Initiative score of 14, I did test with characters that had 13, 14, and 15 and in every tie bot acted first.
Could be the order the characters are placed on the map
I tried to approach a Sentry Bot in the tutorial from 4 different directions and in every case he always acted first on a tie.
or maybe alphabetical order
I don't think so. I tried 3 different characters named "Test", "A" and even "Sentry Bot". In all cases my PC acted last.
I guess I will just count ties as acting last then. Still it would be nice if any of the developers could comment on this.