Ok, having spent more time troubleshooting, I believe the steps to reproduce are as follows:
Have a wide-gamut monitor
Create a custom color profile for the monitor and have Windows use that profile
Set gfx.color_management.mode in Firefox to option 1. (The default is 2.)
Continue with steps in previous post
That's a pretty narrow set of circumstances, but maybe someone else can reproduce.
Anyway, by making some tweaks, I think I have it working now.
Just as an extra note to this discussion, I have a barometer on my phone and a basic small widget showing a pressure graph. Alongside it I have a meteogram simplified to only show a forecast pressure graph, so that forecast vs actual trends are easily comparable.
The meteogram also has the actual QNH (local pressure converted to mean sea level pressure) from the airport a few km away so that is also useful to calibrate and confirm the phone sensor.
They all end up being within 1-2 hPa of each other, so that is good, but seeing the trend is helpful of course. Here is a screenshot of when the Low passed over today:
The Barometer graph (actual pressure) on the left is showing the last 72 hours, and the Meteogram (forecast pressure) on the right is showing the next 36 hours.
Actual from the phone of 1001.99 hPa, actual QNH from the airport of1001.xx hPa (decimals not shown), and forecast of 1002.5 hPa :)