keyof doesn't produce the expected union

[vivdesign]

Hello,
first of all great job with the library.
I recentrly updated from 0.56.0 to 0.60.0 and suddenly keyof stopped resulting in an union and I don't understand why.

This is what the typescript interpreter in vscode gives me for the property

but after generating the schema file I get
"op": { "type": ["string", "number", "object"] },

I'll be very greatful if anyone can help with this.

PS. I also tried 0.64.0 and the behaviour is the same