Fallacies usually refer to a fault in an argument in deductive reasoning. So you have a set of premises (usually a general rule and a specific application) and you derive a conclusion based on these premises. Now if the premises are true the conclusion MUST be true as well (validity) and if the premises are indeed true than the conclusion IS also true (soundness).
Now all fallacies are non sequitur (=does not follow) meaning something somewhere got wrong and the conclusion does not follow from the premises. And as deductive reasoning works with claims of absolute truth value, just 1 counter example suffices to disprove the argument, even if it's just a theoretical one.
The problem is that we have a really hard time even determining the truth value of pretty much ANY claim in the real world. So this kind of thinking is usually limited to the abstract and theoretical where we take certain things for granted arbitrarily and then reasoning our way from there.
So if your commentator indeed makes the claim that P is always the only correct approach, then not P is NOT AN OPTION.
For him to pick not P would necessitate to give up P and take a severe hit to his credibility and claim of expertise given that he wholly put himself behind a claim that was false. Implying that his credence in the claim was more based on confidence and charisma than facts.
If he instead just happens to pick P all of the time, because he didn't come across a case where not P is the better option, then there might not be a contradiction at all. On the contrary it's a fallacy to assume he must pick P all the time just because he picked it all the time so far. That a trend continues is an inductive argument and depending on how long the streak had been, it's a weak or strong one, but the trend alone does not necessitate that it continues. See gambling...
Now to be fair if he is making the claim that "I always pick P, so me picking not P is an indicator that not P is correct", is again an appeal to a trend in that case "I was right all the time, so I will continue to be right" or "You liked what I did so far, so you'll also like my new stuff". Which again is probably likely (to varying degrees), but not a certainty. Like anybody can probably name counter examples of people "selling out", changing their style or even just figuring out that what you liked about them was never what they considered to be an essential part of themselves.
There's also another case that a 3rd party says that about them. So "They've always said P and now EVEN THEY say not P". Where the implication is that of a phase transition where the amount of counter evidence finally came up to a threshold where even the most fervent deniers had to give up their resistance.
Though again that is just one interpreting narrative, but there are countless of other reasons why they would have changed or not even changed their position (see examples above).
You could also make the argument that a change in one's core positions takes more energy/thought/incentives than a continuation of holding a position given our psychological biases. So a change in position could actually mean that person has applied a higher than average level of thought (with respect to THEIR average) to that topic (whether that means much is a different topic). But that's also just a heuristic not a hard and fast rule and you can find counter examples like people who faced incredible harm and backlash for holding a position or opportunists who regularly move positions depending on what's better for them now or the incentive to change might not have come from arguments and deep contemplation, but from money and/or coercion.
It might work as a heuristic in some cases, but it's not sufficient for a general rule. So in the first case it's about the success/failure ration being above 1 in the 2nd case it must be correct all the time.
Also for completion. Being wrong and admitting that is not necessarily a failure or a personal flaw of one's judgement and a lack of expertise. If you deal with the facts as they unfold it's an inherent risk of the craft to be wrong and not necessarily a problem of the individual. However how much that effects them personal depends on how well they sourced their claims, made their reasoning transparent, whether assumptions were labelled as such and whether uncertainties were identified and made transparent, made clear that he's arguing with heuristics rather than general rules and so on. Then they've displayed enough skill and expertise to argue that they weren't the part in the system that failed.
While if they do the opposite, exaggerate claims, merge facts and fiction, leave no room to follow their reasoning and generally make themselves some kind of magic 8 ball that just happens to be right more than wrong, then long list of failures or a critical failure, will lead to them being viewed as defective at what they do. Regardless of whether they actually were the part in the system that was actually defective.