This started happening a few releases ago, I'm just getting around to it. I have a tab group where one of the directories is set to be reverse sort order. This worked for a long time. Suddenly, no longer.
It's just a Download directory, three columns, sort descending order by modified date. No matter what I do I can't get the modified (or other columns) to sort in descending order.
If you hover the mouse over the lock icon in the toolbar (hover, don't click), some information about where the current folder format comes from should appear. What does it say for the affected tab?
My guess is that the Sorting Options part of the tab's format is not turned on, so the settings within it are not being applied. Have a look in the GUI by editing the tab group, then selecting the tab and clicking the Edit Format button:
(I think there was a bug a while ago where those sections would still override things even when turned off, which is probably why it was doing what you wanted before.)
What's on the Columns tab, if that's also turned on? Is that specifying the correct sort column? (And is it specifying just one sort column, or multiple?)
If you turn off the Reverse Sort option in the format, do you get what you want then, or does that have no effect?
The date columns are reversed-sorted by default, so you get most recent at the top on the first click. That might be confusing things. Assuming the normal settings under Display / Fields:
Don't worry, there are so many layers here that it's so easy to miss one. And we did fix some things about how Folder Tab formats override other settings a few updates ago, which could be why things changed out of the blue.