The original Mention object allows to set ta date as follows:

mention.Mention.Date.Date = new Date { Start = ... } 

This results in a serialized JSON that has 1 additional date property in the middle. Notion validation returns the following:

.mention.date.start should be defined, instead was `undefined` 

I introduced a hotfix that skips the additional Date property:

public class MentionPatch : Mention { [JsonProperty("date")] public Date Date1 { get; set; } }