Unusual yes, but probably explainable.
Firstly did you test in the same set up with everything calibrated?
Secondly is it possible that you feel a little more fatigued after 3 weeks of training compared with when you tested initially?
Thirdly – what about other metrics such as HR and your perceived effort – where they significantly different as well?
Taking these things into consideration can give you a better analysis of the discrepancies. Pick the power zones that feel right in your training – remembering that FTP is not an indicator of success per se in Ironman Training – being able to stay consistent in your training and not getting burned out before race day is though.
Testing is great for establishing training zones, but over testing can lead to frustration and exhaustion due to the effort required – particularly all out FTP tests.
I would recommend going with the FTP that feels right in your training and not changing it too much over your training block unless things start to feel too hard or too easy across a few workout types.