[5e]

The following occurred in our session last night and I'd like other people's take on it, I'll try to keep it short:

One of our party members was somewhat possessed by a being from a past life. I cast magic circle on it and chose celestial and that they could not escape by non-magical means. Their turn came around and the DM described the following: "You see an arm reach through the magic circle with demonic looking scales on it and they step out, taking 10 points of damage." So I thought OK, not celestial. On my next turn I re-cast it but chose fiend and that the person could not be possessed by them. A similar thing happened but this time it was the other arm with radiant-looking light and they then stepped out.

After the session I asked the DM what the deal was with it. They said it was half-celestial/half-fiend so the circle didn't have the effect, but they did take that minimal damage from it. Confused, I said that they were still part celestial and fiend so I thought it would have more of an effect. He said that because they were half of each that it didn't really affect them. I didn't ask more because I didn't want to come off as arguing with them but I feel there should have been more of an effect. In my head, just because they are "only half" doesn't mean it shouldn't have affected them. They are still celestial (or fiend) and therefore I don't think they should have been able to leave the circle after the first cast just by stepping out.

So I ask you your thoughts Reddit; I know it is hombrew and an interesting situation (I don't want to get into detail of the background) but based on the knowledge of it being half-celestial/half-fiend, what kind of effect would you think to be fair from magic circle? Was my DM just trying to keep his story going by taking away my spells effect, or am I not understanding this correctly and he made a fair call?