The workaround is to use "Heart Rate - Sport Gear + Wear" (SGT). This app can use the built-in heart rate monitor in many Android Wear watches or Bluetooth chest straps. The key is to make sure you have the "Fitness Activity" type set to "Unknown" and enable "Google Fit: Export session data".
This will work regardless of the app that creates the activity/entry in Fit (it will even work for Google Fit's auto-generated activities). I use this with Strava, but also when recording walks with the Google Fit app, to confirm I'm working as hard as I intended. I have also used this to add heart rate data to stationary rides recorded using Wahoo Fitness -> Strava -> Google Fit.
Once you finish recording an activity in SGT (as "Unknown" activity type), it will sync the heart rate data to Google Fit without creating an activity in Google Fit. Once the activity is created by Google Fit/Strava/etc, it will automatically pickup the heart rate data. I doesn't matter if you create the activity or sync the heart rate data first, it will get picked up either way.
Here is an example of data from a walk recorded using the Google Fit app before SGT has synced: