There are at least
two tribunals that took place during the current Council term in which the Arbitrator changed the rule under which the infraction was given and the result was an overwhelming majority in favour of Uphold and from what I recall little-to-no complaint about it from users when the tribunal was published.
I don't say that to call anyone in particular out, but it's easy to focus on times when you think the infraction was illegitimate and the Arbitrator's reasoning was faulty or at least a stretch, and ignore the occasions where it worked out. I do understand concerns about 'infraction-hunting', especially when a rule like Rule 4 can kind of be stretched to encompass all kinds of posts, and I think it's important as an Arb to be a little cautious about that sort of thing, but I'm not convinced that we'd get better outcomes if decided to never change the infraction basis. It doesn't happen often enough for the change to be significant; you'd just get a few more overturns every year.
And I think it's worth noting the flip side of this. SV staff in my experience get just as much flack from the users and Council about people not getting infracted and 'escaping justice' as we do for people getting infracted wrongly or unfairly. It'll only take one dumb technicality case for everyone to be like "well, why didn't you just infract them under rule X instead?" and get mad about that. Obviously it would be ideal if Moderation simply 'got it right' every time and made Arbitration redundant, but that's not a reasonable expectation to place on them.