Looks like problem in serialization/deserialization JSON mechanism - even with connected Newtonsoft.Json it uses it's own JsonUtility.FromJson which has some restrictions. I made it work when in TonSDK.Connect/Provider/Models.cs changed all the properties to fields and all the field names made as camelCase. Now it works well for me even with High Stripping Level.
Looks like problem in serialization/deserialization JSON mechanism - even with connected Newtonsoft.Json it uses it's own JsonUtility.FromJson which has some restrictions. I made it work when in TonSDK.Connect/Provider/Models.cs changed all the properties to fields and all the field names made as camelCase. Now it works well for me even with High Stripping Level.