Gareth, great question. This may be an example of the “observed” and “unobserved” performance phenomenon. Athletes perform better when being monitored or held accountable in some way. An individual threshold test, with no external party to observe, will not provide the same performance as a result that will be public in some way. Your Zwift Academy is not a race, but it certainly has a higher level of 3rd party accountability than your previous individual and somewhat anonymous result.
We see this all the time with athletes who have rFTP X when they do a 20-minute TT on their own, but are 10% higher when doing a 20-minute 5K race.
So, the answer is: can you maintain the same level of motivation in every training run that you had for your Academy run or race? If so, we should use the new 330.
Also, these two results are close enough. 323, 330… not a huge difference if you pick one or the other.