Advice. Am I overreacting?

Yesterday my fiancé of 4 months was watching my kids (3&5 yrs old) in the waiting room of the doctors while I was back in a room. He told me they were acting out, not listening.

When we left the doctors office I walked out first and he walked out behind me with my youngest son. I turned around at the exact time he spanked my son on the butt with a small book he was carrying.

I confronted him about it and his response was, “it was a moment of weakness, he was not listening”. And that he also “thought he was at the level where that was okay to discipline like that”. I am not happy as I feel we have never agreed he could discipline like that. I also feel that was a moment where he got his frustration out and not a moment of teaching my son right from wrong.

I’ve never seen my fiancé angry in any way and he is generally very good with the boys. Im worried that this could be a huge red flag I need to not ignore. Also concerned that he did it behind me thinking I would not see him.

What would you do, say or think?