Lightning keeps starting the week on Monday

[DVB]

I've asked about this before, but wondered whether anyone had found a solution yet.

On calendars I like the week to start on Sunday. I have a paper monthly calendar which has the same info I have on Lightning -- with the week starting on Sunday. But every few weeks Lightning changes to the week starting on Monday. I have to go to Settings, reset it to Sunday (there's an option to start the week on any day you like), close Tb, relaunch Tb and check Lightning, and it's starting on Sunday again. For a couple of weeks. Then it reverts to Monday.

Other settings remain as I set them, even when there's a new version of Tb (except a few weeks ago, inexplicably, it completely messed up the columns I have showing in my Inbox etc) -- so why won't this setting stay as I set it? Is there no way of forcing it to stay put?

Advice welcome!

[mtalbot]

Does the calendar reset only happen when an update for Thunderbird is installed?

I know that the week starting on a Monday is the current International standard but I, too, like my calendar week to start on a Sunday so a fix would be good.

[DVB]

Does the calendar reset only happen when an update for Thunderbird is installed?

Tbh, I'm not sure. It tends to stay at Sunday for a week or two, then revert to Monday.

[DVB]

And this morning it's back to Monday. So far as I know there hasn't been an update to Tb over the weekend.
It's bl**dy annoying. Surely there must be a simple fix for it.