Barometer indicator does nothing, never has

I've only seen it horizontal.

Must admit I'd never noticed it before but having taken a look now, it's horizontal. 

Mine has never ever shown anything but horizontal 

Does the barometer glance show variation? Does tge altimeter work? If not, the sensor may be faulty.

The baro trend arrow on my 955 (mostly very similar except display) works fine, however I use a CIQ watchface  (SHN TxD II-E). Stock watchfaces may implement the baro trend arrow differently.

The arrow is still flat most if the tine, the change in ambient pressure has to be fairly large and happen quickly to show up.

Yes, the glance shows my 6hr baro history just fine, alerts go off when a storm or big front approaches, altimeter is spot on, the tren arrow refuses to budge. On stock watch faces and the ABC the trend arrow never ever is anything other than flat

I guess that it is likely a watchface issue (it probably does not update the datafield). You could try a CIQ watchface to see if that resolves the issue. And/or report it to Garmin, in hope that they will fix it.

No, it's just not working natively. It doesn't work on the ABC page too, so it's simply not being updated anywhere and the third party watch faces use their own data icon, I know as I've tried this making my own face and it works fine. Reported several times without either acknowledgement or fix

Not even acknowledged, that's sad. I'll set a stock watchface on my FR955 and see if that has this issue too (probably) and report it as well (you never know, the more reports the better)