Pace discrepancy when using HRM Pro for pace.

That’s the way it is. Don’t use it. It’s a very inaccurate way for pacing. 
Would only be my fallback indoors if you don’t have a footpod. 
The 9.00 will come from the watch. Either GPS for outdoor or Accelerometer if using indoors. 
to have everything match you also have to use distance setting. 

I think you need to contact support or maybe directly Chris here regarding your problem if this is important to you. As this is new feature and apart of watch you need to have expensive strap I assume not many people use it currently. I also love to have something that give me instant PACE especially that PACE information from Fenix in the forest is crap. Some time ago I try to use Stryd that claims to be most accurate and I had not nice surprise after finishing Half Marathon (certified track) when according to Stryd my distance was 1km longer. I contacted support from Stryd but they were keep asking me to perform tests on track ignoring fact that I delivered few obvious examples of inaccuracy. At the end I gave up on that topic. My outcome was to not use any food pod as you never know what results you will get as discrepancies may be different for different PACE.  

I have the same issue. HRM-Pro over estimates the real pace by 0:30 sec per kilometer. So it's completely useless. In my experience, if you want accurate pace the best is to use a footpod, multi GNSS position - or if you can, dual band GPS.

I must say that it is really accurate for me, on a treadmill, I guess as long as it has had the time to calibrate it self.
I wasn't sure before, but recently i ran on a treadmill in a lab where they calibrate the treadmill, and it was spot on.
On the gym treadmill it does not match up the speed on the band, but it match the perceived effort and pulse very well so i go on the HRM plus pro.
I also notice that the pace easily manipulated if you start to change the stride and so on, but if you run as usual, it's very good.

Alas I gave up using pace from the hrm Pro. I also noted that the same issue arises when I use my old garmin footpad.

I had a similar (maybe ?) problem using a footpod and a F7X-SS. The pace was "strange".

The new firmware 10.43 seems to have fixed the problem for me. In the release notes, they say something about pace and footpods/HRM-Pro. Hopefully it fixes the problem for you too ?

There seems to a few ways to handle these issues,

have you tried calibrating in a few different ways?

https://support.garmin.com/en-US/?faq=8B0cpkix4E8kJHHhs2OzY6

one strange thing is that my HRM pro + used to ask for calibration after a treadmill run, but it doesn’t anymore.

I have without success.  Contacted support who suggested a hard reset of watch that I don't fancy as data will be lost.

That's interesting, and I totally missed it ! Thanks.

It means if you set the outdoor pace to always, the calibration never occurs - so the displayed pace is calculated from inaccurate data.

I have reset the calibration data, turned pace to indoors only and will check again when calibration is complete.

Thanks for the heads up !