jumping in:
Theoretically the lower functions can arise in stress, causing an NFP to look STJ and vice versa.
yeah...do you buy that theory tho?
i honestly don't...i mean...a stressed enfp...can be all crazy and te like with completely inaccurate si shit going on but it doesn't look like an istj who's in command of those functions.
and...the only time an istj might seem like an enfp is after many cocktails or happy pills of some sort.
in my super expert opinion
i've noticed that theory seems to have a lot of different versions:
"in times of stress, you.."
1. switch all letters - reverse the order of the 4 primary functions (ENFP to ISTJ).
2. reverse the I/O polarities - tackling one's shadow functions (ENFP to INFJ).
3. switch the order & the polarities - tackling one's functional opposite (ENFP to ESTP).
i'm not sure about if i buy any of those, but if shadow functions are real, then the last one seems to make the most sense, because i do think that there is a relationship between the behavioral attributes (P/J & I/E) and the functions we act on, and i noticed that when people aren't dealing with stress in a healthy manner they tend to push their behavioral attributes to the extreme (P's become more chaotic, Js become more uptight, introverts become more reclusive, etc).
to the OP: yes, over the years i've noticed my mother moving from ENFP hippydom to embracing the little uptight ISTJ inside - this has only really started in her 50s though, and i am pretty sure it just has to do with the late development of the lower functions.