FP4+ shows a lot more grain at EI125 than at EI50.
Apart, at EI125 its tone isn't as clean as at EI80 and EI50.
At box speed, its shadows have less contrast/separation as we're using film's toe.
FP4+ at EI125 can look, with some developers, very close to HP5+ well used: an ISO400 film look, more than an ISO100/125 film look.
Of course FP4+ can be used at EI125: it won't warn us.

Obviously we can place middle grays where they should be even if we expose at EI125, but that doesn't produce the best FP4+ tone or grain.
Metol developers work very well with FP4+, and even D-76, if we expose at EI50, produces results that are very close to those of Perceptol/D-23/Mic-X.
About Rodinal: 1:25 is not necessary. Even for soft scenes 1:50 is more than enough: FP4's native contrast is higher than HP5's.
But IMO it's EI what defines image structure with FP4+.
So, if apart from exposing FP4+ at EI125 (not the best tone or grain), we develop it with Rodinal (which depresses middle tones a bit, and gives big grain), FP4+ looks like a bad ISO400 film, not like well exposed and well developed FP4+, a gorgeous film.