Thank you for the report.
Despite the clear picture that your screenshots paint, for us to investigate and act on that example you provided, we would need more ingame data, preferably a couple of saves before the battle and one during.
The is no ingame logic that would give enemy AI some additional brainpower, as compared to regular, or allied AI. What I suspect happened is that Byzantium units decided their current objective was more important than a battle couple tiles away (once again, in AI vs AI situation, the behavior for this example would be the same), while enemy units where close enough to respond, so joining battle took priority for them.
Despite the clear picture that your screenshots paint, for us to investigate and act on that example you provided, we would need more ingame data, preferably a couple of saves before the battle and one during.
The is no ingame logic that would give enemy AI some additional brainpower, as compared to regular, or allied AI. What I suspect happened is that Byzantium units decided their current objective was more important than a battle couple tiles away (once again, in AI vs AI situation, the behavior for this example would be the same), while enemy units where close enough to respond, so joining battle took priority for them.