This issue is better explained with an image:
After restoring the unison, there’s no reason for placing the “piano” marking in a blank staff, unless it had music in the first rhythmical position. Or maybe it should only show up in the first note thereafter, unless another dynamic marking overrides that at that rhythmical position or before.
The problem is that the “p” is rhythmically attached to the first beat of the third bar. In Write mode, move it to the end of bar 2.
Thanks Leo
Actually that’s what I was going to do, I just posted it as a suggestion for taking in consideration for fixing. The fact that Dorico considers final dynamics as part from the next rhythmical positon is very clever and saves me a lot of time of dynamic positioning that I was used to do in Sibelius, but it comes with these side effects. My sense of “what I think it should be” prevents me to displace the dynamic a 64th back in time 
I agree… there are some odd side effects that I wish could be mitigated. For example, if you select a whole note in the final bar of a 4/4 and add “p<f” or whatever, Dorico creates an additional bar to facilitate the forte. Err, no. Although I understand the behavior and generally appreciate it.
I’ve come right now from having exactly that issue too. I also understand why. With the current dynamic placement logic, where do you attach the last dynamic if the rhythmic position doesn’t actually exists? Nevertheless, it doesn’t look right anyway.
Perhaps in the future (tomorrow?) there could be an added functionality that, instead of creating a new bar for the final dynamic in this use case, adjusts it to attach to the last rhythmic position in the final measure according to the current rhythmic grid.