Slitherine

I thought I understood the mechanics around when and why an army will retreat and where they will go. But a recent experience suggests I don't. So if someone could clarify this for me, I'd be grateful.

I, Macedonia am at war with the Antigonids and their client, the Getae, both AI controlled. The screenshot below shows the situation after the events in question.
On the previous turn Antigonids and Getae were besieging the region of Thracia with two armies (one each), my army was in the Hellespontus with orders to land in Thracia. I can't show a screenshot of the previous turn as it's a MP game, so I can't go back.

My army of 231 combat power slightly outnumbered the combined Antigonid/Getae armies. One Antigonid army began moving towards Strymon, but did not clear the region before I landed and beat them handily, suffering zero casualties to their loss of 6-8 units. The Antigonid army did not retreat however. They continued towards Strymon, despite not outnumbering their enemy, but rather being outnumbered at the beginning of the turn, and even more so after that battle. The Getae army retreated to Mesembria.

Then a small Getae army arrived (less than 50 combat power), triggering another battle, which the Antigonids were drawn into. I won this battle too, inflicting @10 units casualties on the combined force of Antigonids and Getae. At the start of this battle I outnumbered the combined total of Getae and Antigonid forces, hugely so after the battle.

The remains of the Getae army retreated to Mesembria. The Antigonid army continued to march to Strymon. I thought it was weird that they would retreat into my territory, since I didn't believe this was possible. But they were not in fact retreating, they arrived in Strymon, and continued marching straight into Migdonia, where they attacked the garrison and defeated it. Taking the region. This is not a major setback or anything that will affect the outcome of the game, but I would like to know why it happened so I can see it coming in future.

So my confusion is, why didn't the Antigonid army retreat after being beaten by a superior enemy (superior in both numbers and combat power, and probably any other metric you could measure them on), but instead continue its move order, not once but twice?

Also, before this turn the Antigonids had at least 3 possibly 4 units of Silver Shields. How? I thought this was a unique unit? I have managed to have 2 myself previously by absorbing another Greek state who had one, but doing this 2-3 times seems improbable.

I can answer with some certainty the 2nd question, it's an Antigonid feature, they are the only ones that can build more than one Silver Shields at once, but with an increased cost.
As for the initial question, has the game told you it was defeats, or simply draws?

Thanks for clarifying the Silver Shield point. I should pay more attention to my enemies abilities.

The game listed them as victories for me, I got the pursue phase and inflicted additional casualties there.

Had another weird issue with another MP game. Playing as Carthage, Mauretania invaded and occupied Anas in southern Spain, I sent an army in and besieged their army in the fort. A turn later I ordered an assault on the fort, then on my next turn that Mauretanian army marches out of the region without a fight, my army occupies the fort without a fight and liberates the region while the Mauretanian army invades into Baecula to the east, occupying it. How did a besieged army get out of a fort and invade an adjacent region without fighting the besieging army? Is this some kind of timing issue between when my assault and the Mauretanian move actions trigger?

The only case in which I ran into such a situation was when obtaining a victory against overwhelming numbers: the Persian DLC is especially handy to reproduce it. When more than a hundred squads invade a region in which I win an ordinary 8v8 or the like, their troops ignore ordinary rules and just keep travelling!

Yeah, I'm familiar with that mechanic, but in these situations I outnumbered the enemy, which is what led to my confusion. I'm tying to see if there is a mechanic I'm missing, or should I be submitting a bug report.

If I did submit a bug report, I'm not expecting much would be done at this stage in FoG:E's life cycle, since they are minor problems, but hopeful that steps would be taken to ensure they didn't reappear in FoG:Kingdoms whenever it launches. Since my understanding is they will share a degree of their code base, but I could be wrong.

It's theoretically possible, if the destination only cost 1 MP and the sortie manages to enter the new region before the battle sequence trigger. It would also have to not be repulsed by the besieging army, so the odds should be lower and lower the bigger the besieger is compared to the besieged.
I know it might feel odd to make a sortie and manage, at times, under certain circumstances, to avoid the besieger, but making this different might add some extra time to the processing loop, so I'm reluctant to change anything in this tight loop.