I've had a couple of days being ill, leaving the Watch off overnight to avoid the miss-counting, and spending most of the daytime on the sofa. However, on both days I've met the 12 hour stand goal. The second day I kept having a look on the Watch and I noticed that often, in the bar chart type visual below the combined circle graph, an hour had a faded out blue line but had not counted it in the goal (I've not noticed this in the previous watchOS - but I was not looking either, so it may well have been doing it then too). However when the hour was almost up and I received the notification to get up and walk about... just looking at the Watch and reading the message counted and more often than not push me over its measurement, turning the line to full blue and count the hour as meeting the stand goal.
It's looking like a software issue with how it interprets the hardware used to measure the stand metric. I've not been well enough to see if my odd activity shortfall was a one off, or something else going wrong.
I guess we are all hoping Apple engineers actually look at these forums and realise there are other issues with the now pulled update (missed or ignored via beta testers), and will fix them in the replacement build of the pulled software.... oh and allow that new software to overrule the versions installed that worked and did not brick watches.