Sent strings are malformed in the MeasurementAnalyticsClient.AdjustUriBeforeRequest method, when a string contains chars = or &.
E.g. sending
AutoMeasurement.Client.TrackEvent("My Action", "My Custom Category", "My Label id=345");
results in sending label="My Label id" only.
I've solved problem either by using UriFormat.UriEscaped, or fix GetQueryParameters method by q.Split(new char[] {'='}, 2). (Just one of this fix solved problem.)
Note: Such Uri construction, parsing and again construction is often source of problems. (My experience from several other applications.) Better to avoid it. Just construct the right Uri once and send it.