Dolmenwood/OSE and kiting in combat

I just finished the first session of a new Dolmenwood campaign I've started up, and our group found a weird interaction with the combat rules that left us stumped.

The party had been ambushed by a group of five crookhorns. One of our guys, a thief, was sitting in a tower during the ambush with his musket (same reload mechanics as crossbow) and started shooting at them. Two crookhorns broke off from the group to chase him down. Both had a speed of 30'. The thief jumped from the tower and ran along the battlements, moving 30'. Then he used his action to reload. The crookhorns ran towards him, also moving 30'. He then ran another 30' and used his action to shoot again. The crookhorns chased him again, 30'. The thief slid down a ruined section of wall and ran across a field, 30'. He reloaded...

At this point, we all realised the problem: Without some kind of barrier to prevent him from moving, there was no way those crookhorns were ever gonna catch up to him. They would both move 30' each round, and the thief would keep shooting at them until they died. We all agreed that this made no sense - someone moving and shooting would almost always be slower than someone just running - but we couldn't find anything in the rules that would make the crookhorns faster than the thief.

The closest we found were the fleeing/pursuit rules, where someone fleeing can run up to three times their speed, and someone pursuing can do the same, but that didn't seem to apply here, since the thief wasn't actually trying to leave the encounter, and so he wasn't fleeing according to the rules. We couldn't find a rule allowing you to move faster during combat, only outside combat, so we figured that what happened was correct according to rules as written.

For now, I resolved the problem by just encouraging my players not to abuse this mechanic until I found a good ruling for it. Ideally, I want to avoid a situation where someone of equal speed can just kite forever, while still allowing someone genuinely faster (like a mounted archer) to kite if the terrain allows it.

So I'd like to know: Has anyone had this issue before? How did you resolve it at your table?