Getting active time from Google Fit Rest API - rest

Question

I am struggling with matching the "active time" returned by Fit REST API with reality.
As an example - on 12/14 I had two walks, about 45 minutes each. The api returns one of them as type 7 ("walking" - right!) and another one as type 0 (in vehicle - wrong!). However, Fit app shows both as "walking", so it apparently uses a different data source.
I checked some other days and on these days, the session with type 0 is indeed a valid "in vehicle" session.
I tried all aggregated data sources that return com.google.activity.segment. Most of them are empty, I've found data only in merge_activity_segments and platform_activity_segments (and they seem to be identical).
Google's docs have a caveat about delay in data sync, but they never specified how long this delay is. The data I am looking at is about 24 hours old - if their sync is that slow, then this API is more or less unusable.
I am using the following POST to https://www.googleapis.com/fitness/v1/users/me/dataset:aggregate
{
"aggregateBy": [
{
"dataSourceId": "derived:com.google.activity.segment:com.google.android.gms:merge_activity_segments"
}
],
"endTimeMillis": "1481788800000",
"startTimeMillis": "1481702400000",
"bucketByTime": {
"period": {
"timeZoneId": "America/Los_Angeles",
"type": "day",
"value": 1
}
}
}
For reference - activity types: https://developers.google.com/fit/rest/v1/reference/activity-types
Has anyone been able to retrieve activity time from Fit's REST API that is correct? Any suggestions?
By the way, steps and calories seem to work fine - just aggregate the following datasets:derived:com.google.calories.expended:com.google.android.gms:merge_calories_expended and derived:com.google.step_count.delta:com.google.android.gms:estimated_steps
A side note - it is probably the worst documented API from a major company I have seen.