May be a bit off topic, but I also wonder how a behavioral source with P should behave. If I assume P is instantaneous power (to sink from a source), here are two examples that in LTspice to think of
Example #1 : Source V1 is a 0.5V sine wave offset to 1V, and B1 sink 100W.
In this example, instantaneous power not always at 100W
Example #2 : Source V1 is a 0.5V sine wave, and B1 sink 100W
In this example, not a single instantaneous power can reach 100W
P acts quite weird in LTspice, it seems defining behavioral current source can make thing more clear if to sink user defined instantaneous power
I = {instantaneous power} / {voltage at Bx pin -}
