The formatDateTime reference points to the single format specifier documentation where you can see that each single character specifier has different outputs depending on a given Culture (eg. en-US, nl-NL, ru-RU).
Of course we can use the custom format pattern as a workaround, but even some of those options rely on culture (eg. ddd, MMM).
Please add Culture as an optional parameter to the formatDateTime function in Flow, so we can better control the behavior of the formatting.