Hi,
The DynamicText's Y-offset controls its position relative to its baseline, which is the VerticalAxisGroup of the Dynamics context. Without \break, the VerticalAxisGroup contains two dynamics, "f" and "p", with "p" offset compared to "f", which forces them to be at different positions. With \break, the "p" is alone in its VerticalAxisGroup. It is properly offset compared to that baseline, but then the baseline itself has to be spaced within the rest, and this spacing cancels the effect of your tweak.
HTH
Jean