I'm of a differing thought about this. In a herd situation, this mare would kick out, and either be disciplined until she realized her place in the herd, or would become dominant and therefore a bully, as she would keep repeating this until a firmer horse was introduced into the herd.
AND, have you made absolutely sure there is nothing physically wrong here, as in peritonitis, spinal problems, "mare" problems within? And that will mean more intensive tests than a vet having her walked around and looking at her, and doing a few tests just as temp taking. IF, and this is a big IF, she has something going on internally? You need to fix that first of course.
We had a TB colt, that was a crypt, but he was put into training. Ran pretty good for little bit, but then would show pain that got worse. X-rays revealed that the missing testicle had lodged right up against his spine in saddle area. It had migrated that far, or been there all along? But point is that without a clear look at what is happening inside? You may be seeing the signs of a horse in unremitting pain too.
And you also didn't say what breed?
The pain issue addressed, let me go on to say.
I understand the wonder about forcing the issue, and if it was just the tail twitching? I'd ignore it, but to me, the longer you let this sign of disrespect go? The bolder she is going to get. Right now she is at the "I DON'T like you telling me what to do" and is testing the waters, so to speak, to see just how bold and brassy she can be would be my thoughts.
Also, this will escalate until anyone walking by her, or around her could be fair game. For instance, in the warm up ring, walking her back to stall, or whatever, and she could add more to it too.
And there is a big difference in horse's mind, as to "deserved correction" and undeserved, (although with TB mares? They will hold a grudge about being gotten onto for behaviors and wait for timing.) And many a horse has been overcorrected by person, who would not get tough enough at first to get point across, but just kept flailing away.
With aids? Gentle, gentle, firmer FIRM. With correction? FIRM, FIRM, FIRMER! That stops the escalating anger and frustration level when it is fixed simply and firmly the first time, rather than human beating themselves into a frenzy. And not coincidentally the horse. THAT leaves a mark on both. A firm correction, one meant to end the problem quickly would have worked.
Pain ruled out? I don't think being kind/ignoring it/work is what is needed. I'd either tie leg up when working with her, or put kicking chains/hammers on her, and let her correct herself, or I would keep shank on her and correct her as needed.
To me, when you "move" a horse like this for correction, it may actually be what she is wanting, to do something, and working is adding to it, if work is a correction. You want her to stand still, so I'd find a patience tree, add some anti-kicking devices, and let her stand tied for several hours, and I would continue to correct her as needed.
Horses make me a better person.