This is a really great question with a very complicated answer, however one that is very close to my heart. I hope you will take the time to consider my response carefully before arriving at your own decision.
I am a HUGE believer in free work. So much so that I begin work with every and any horse from that position. By free work I mean a small square arena. Not a roundpen. Not join-up and not NH carrot stick liberty work. I don't do it because it is pretty. I do it because it enables me to control ANY horse. Horses that otherwise are shut off or cannot be controlled with halters and ropes. Horses that trainers cannot train.
However when I teach others how to do this there is a very important process. Strict boundaries and a specific purpose, even if the work is free flowing and flexible. I encourage the horses to express themselves but it is NOT play. Equally as much work goes into developing the handlers own body awareness, balance and muscualr control as the horses.
Horse body language is exactly that, a language. It is very precise, very complex, very detailed, with many nuances and even 'individual accents' just as individual as personalities. It is no different to our own language just that we cannot rely on sound to confuse it or reinforce it.
Unfortunately none of the big trainers spend any time or have any real understanding it would appear of horses attempts at interaction with them. 'Its all ears back bad, ears forward good'
What concerns me greatly is that due to the NH liberty work displays, along with videos of people like Alexander Nevzorov or Frederic Pignon, many many people think its okay to run out and 'play' with their horses. Yet they have no concept of what either Nevzorov nor Hempfling are actually doing. It is not play.
So the answer is it depends. I see many videos on you tube of girls having their ponies and horses chasing them around the arena, jumping etc. Often following clicker based commands, etc etc. That is bad. It is dangerous. Very dangerous. These horses will often not display any of the signs that we rely on for dominance such as ears back. Yet their body language makes it clear they are being dominant. The moment suits them so its fine, but if that moment changes - accidents happen. A girl on a local yard had a broken cheek bone, and several lost teeth (including some punched through her lower lip) and was lucky to be alive. I know another girl who has a steak sized chunk of flesh missing from her thigh from a horse bite under similar circumstances.
The thing is I NEVER play with horses. They do and can express themselves, but chasing play is not allowed. Anytime they are expressing, bucking, rearing or anything else is fine, but at a set distance, and the second I raise a finger they STOP DEAD.
There is a set purpose behind what I do. It is about physical development, and recognition of the body shifts that will become the weight cues that will transfer to their backs.
When you ask a horse to chase you are instigating a process of physiological changes in the horse that affect how it feels. In other words you are giving them power over you. That is a dangerous thing.
So the question really is (and unkown without seeing it) is the horse chasing or are they being driven? That is the real clux of whether this is relatively harmless or potentially life threatening. If it is the former I would discourage it instantly.
Take care and please take this in the spirit it is intended.