trying to fix this locally, i ended up just having the inner private lists get serialized/unserialized. i'm not sure if it's the best solution, but it was the least complex one

//[JsonProperty]
[JsonIgnore]
public IReadOnlyList<TimingControlPoint> TimingPoints => timingPoints;

[JsonProperty]
private readonly SortedList<TimingControlPoint> timingPoints = // ...

unfortunately, this doesn't seem to work on groups since it doesn't look like BindableList<T> is supported for json serialization/deserialization

Newtonsoft.Json.JsonSerializationException : Cannot create and populate list type osu.Framework.Bindables.IBindableList`1[osu.Game.Beatmaps.ControlPoints.ControlPoint]. Path 'control_point_info.groups[0].control_points', line 76, position 27.

since it doesn't look like BindableList<T> is supported for json serialization/deserialization

that's incorrect, the concrete type (de)serialises just fine, just checked.

the issue is the interface spec, newtonsoft doesn't know which implementation of IBindableList<T> to choose when encountering the groups members. this presumably only works for built-in collection types by some sort of hardcoding on the newtonsoft side for convenience.

yeah, that makes more sense. i just assumed BindableList<T> was the problem since serialization worked just fine on the bare IBindableList<T> and i didn't read the exception closely enough

Does the same solution not work for the groups case as well? ie. moving the JsonProperty specification to the non-interface private type.

yeah, that solution works for groups. i just forgot to do the same to ControlPoints in ControlPointGroup in the original comment. after doing that, i get this:

Newtonsoft.Json.JsonSerializationException : Could not create an instance of type osu.Game.Beatmaps.ControlPoints.ControlPoint. Type is an interface or abstract class and cannot be instantiated. Path 'control_point_info.groups[0].control_points[0].time_signature_bindable', line 77, position 38.

doesn't look like applying [JsonConverter(typeof(TypedListConverter<ControlPoint>))] to controlPoints works either, unless i'm missing something here

Newtonsoft.Json.JsonSerializationException : Error setting value to 'controlPoints' on 'osu.Game.Beatmaps.ControlPoints.ControlPointGroup'.
  ----> System.ArgumentException : Object of type 'System.Collections.Generic.List`1[osu.Game.Beatmaps.ControlPoints.ControlPoint]' cannot be converted to type 'osu.Framework.Bindables.BindableList`1[osu.Game.Beatmaps.ControlPoints.ControlPoint]'.

It can probably be made to work with a bit of refactoring. I believe the remaining issue is that you are trying to apply it to a BindableList but the TypedListConverter doesn't immediately support this.