expired_access_token in .NET API

I'm using the .NET Api with refresh tokens.

I get the authorization uri and the user gives permission.

var authorizeUri = DropboxOAuth2Helper.GetAuthorizeUri(OAuthResponseType.Code, ApiKey, RedirectUri, state: state, tokenAccessType : TokenAccessType.Offline, scopeList : scopeList, includeGrantedScopes: IncludeGrantedScopes.None);

I then get the token. The response does include a refresh token.

var tokenResult = await DropboxOAuth2Helper.ProcessCodeFlowAsync(redirectUri, ApiKey, ApiSecret, RedirectUri.ToString(), state);

I initialize the DropboxClient class with the refresh token.

var client = new DropboxClient(this.UserSettings.AccessToken, this.UserSettings.RefreshToken, ApiKey, ApiSecret, config);

This worked initially however I now get an "expired_access_token" result from the API. Does the client use the refresh token automatically or do I need to handle this and manually refresh the access token?

(I'm using version 5.5.0 of the .NET Dropbox AP).