Very few builds can catch someone who does this, especially if they have the CEL to actually flee and take initiative.
I've seen it far too often where people will engage a party in IC combat, then despite whatever the actual circumstances may be, they'll just bum rush for the edge of the battle square and prepare a flee immediately or very shortly after the fight begins. It's entirely supported by the mechanics, but ICly it makes for silly situations where you can never catch a person if they choose to abuse this and you don't just meta/bring a person with a build specifically designed to catch these people (this would involve either having an absurd initiative score, or utilizing shukuchi > move > QHK to stop non-ma fleeing).
This is only an issue in Serious battles, as spars are meant to be easily ended whenever either side wants. So I feel Serious fights should have some kind of penalty to fleeing or just take multiple turns of using Flee to succeed. I'd say 'make it a round delay' but you know people will just run circles around the map until this arbitrary delay expires and they get their auto flee.
And don't even get me started on Burn Up > Winged Serpent > Flee on Round 0. This has been done before in serious situations, and I sincerely hope people haven't actually done this since. But that alone highlights how silly this mechanic is for resolving how people may try to escape by cheesing how the game's flee system works.
Other ideas to fix how fleeing works in Serious fights are welcome. I understand how CEL/MOV should play a part in how well people can pursue/flee, but as it stands a 1 CEL difference in initiative means that person can and will always escape going by the mechanics as they are. And given you can no longer alter initiative in combat at all, you will have no chance whatsoever to catch someone with initiative if you lack the means to pull them away from the border or remove momentum on Round 0 from your starting position.