The order of my tabs in DO 12 keeps changing while using it. I do not drag them myself, but tabs change position all the time.
I tried using the lock option, but that doesn't seem to help. I also cannot find a preference to change/stop this.
Why does this keep happening?
The tab-lock has no effect on order, but tabs also don't usually move themselves.
You'll need to work out which action is performed when they move.
Are you sure the tab bar isn't just filling up and scrolling to the right/left>
I thought the tab-lock wouldn't be for this, but I just thought I'd try 
I'm unsure which action could provoke it. But I see tabs that I use a lot moving to the back of the tab list (so to the most right position), which is not where I placed them (or dragged them).
Only related post I can find, that isn't about something fixed a long time ago, is this one:
I appreciate this reference, but this does not apply to my situation: there is more than enough room for more tabs on the right (and no tab overflow is shown).
Just an idea. You can make a screenshot of the unwanted order of tabs, and check in your > preferences > folder tabs > tab groups, if that specific order shows up under one of your own tab groups. You can see a preview, if you click through the tab groups at the bottom of the prefs page. Maybe, for some reason, the wrong tab group is loaded.