I'd say such hypocrisy is very common among unhealthy ENFs, though tends to be rare among those who of them who are sound.
Unhealthy ENFJs get absorbed by the external standards of the community and do all they can to meet them, in most cases it means pleasing those around them. Hence, if they have made a plea to one group, and later on it turns out that if they could win the favor of a larger community or more influential people, or whatever it takes to become more liked by breaking the previous promise they'd do it.
So hence, the reason why they are fickle is because they do not have an internal standard to go by and the standards they aling with change.
As for the ENFP, unlike the ENFJ, they dont have a clear-cut external standard to go by. So they can count on no more than fleeting hunches to make themselves feel good, and hence they must depend on the immediate approval of those receiving them for validation. So they'd be going on and on trying to please everyone they come across, as their need for approval is very high. Yet, unlike the ENFJs, as the ENFPs dont have a solid standard to go by, need for the approval to be renewed frequently as again, unlike the ENFJs, they can not keep focus on the people they've pleased in the past. As for the ENFP being fickle, they never really know what they want other than receiving emotional affirmation all the time. Since they need to be the center of attention (Ne-performer aspect, and Fi bent outwards) and lack judgment (no external judgment and Fi inaccessible) they need consistent admiration for others to uplift their self-image and will go at great length to obtain it. Perhaps even as far as going against many of their deeply held personal values. Moreover, they are also likely to be driven by desires as at this point they are ruled by their hunches and do not exercise proper judgment to truly figure out what they want. In situations like these, the ENFP would truly be out of control and it would be obvious to everyone around but the ENFP him/herself. And a salient reason why they cant put limitations on themselves is because they've disregarded reality in favor of their heart-warming fantasies and no matter what happens, they will discard concrete facts and logic so they can befool themselves into thinking that the world is exactly like the way they want for it to be.
Once you shake some sense into them (with great difficulty because they likely will be too fickle to stay focused with you, even throughout the length of that one discussion, or likely wont be able to take anything seriously enough to even begin talking about that..or be too afraid of negative feelings/confrontations..as they need constant emotional reassurance..otherwise their identity would just wither away as they have no solid foundation to stand on), they will be sorry...very sorry..and you'd have to be downright heartless to not believe them..and then again..folks..its not that the enfp deceived you about their motives..they really were sorry..but they're just too fickle for that to mean anything at all..their feelings vanquished right the next day..and damn it..they do it again and you're back to square one trying to confront them...and they are so coarse and unreflective that they hardly understand what they did..and when they get 5 seconds of down time to think..it finally hits them..and what do you know..they again try to lighten up the conversation and sweet-talk their way out of it..anything..anything..just to make sure you dont show disappointment in them and they can carry on doing whatever their left foot commands...regardless of what effect it has on you and others they supposedly care for..right after they got done telling you how much you mean to them and how sincere they are about all of their values..
So the case of a neurotic ENFP will seem even more insincere than that of the ENFJ despite that the latter is more externally focused and has a stronger feeling preferrence.
As for healthy cases of both types, they generally tend to be sincere, yet very often are mistaken for being ingenuine because they are just too smooth for their own good. Especially the ENFP. And the ENFJ for trying too hard to please everyone, most would think they are just too good to be true. Though what vitiates the image of both is the light-hearted approach to life and an external focus, both of which tend to obfuscate the depth of feeling balanced individuals of these two types likely experience.