5x Plus Dropping Ant+ HR

I have been comparing GPS tracks of my 5x plus to a old Forerunner 305 (tracks actually very similar). While testing, I was noticing the Fenix was losing HR on the screen (Garmin Ant+ chest strap). The 305 showed no such issue w/ same strap synced. Looking into it further, on Garmin Connect, you can clearly see that the HR drops out very regularly. Gaps all over the place. The FR305 shows zero drops on the same ride, viewed on GC. Both units mounted on MTB handlebars.

I thought this was a fixed issue.