Since there is no dedicated forum for accessories, I thought it would make sense to post it in a few places.
Today I've got a prompt to upgrade my HRM Dual to version 2.50(previously I had 2.30). So I did that, my Vivoactive 4 is working perfectly with it via ANT+ but all Bluetooth based programs on my mobile, which I've tested e.g. Elite HRV, Wahoo, HRV4 Training, HRV Logger no longer working. Apparently Garmin changed the format they output data or something as now instead of Serial I see 2.50 which is the version of firmware.
I think that the apps developers will need to fix it now, so maybe give it a time before doing the upgrade.
Hey Garmin-Kevin, I guess this may be not your department but are there a way to find out what changes were made in the new firmware to HRM Dual today?
SOLUTION: If you have a problem with non-working bluetooth apps on your phone after the update, check bluetooth settings on your phone and see if you have paired HRM Dual, if so just try to remove(forget) it from paired devices, this step fixed the issues with apps for me.