Yep, just like you've seen it in the title, when selecting 1 province you get a timer of how much it will take for the unit you choose to be completed, example:
However when you select more than 1 province to produce all at once, the timer disappears, example:
Now one could say that those 2 provinces have different, in this case, factory levels, buuut that's not the case, this are the 2 provinces I selected:
For those of you who are curious, what happens if you select a third province with a different factory level? Well, you guessed it right, the timer doesn't appear either.
Workarounds for this?
Well, when the factory has the same level, just add the normal timer.
When the factory has different levels, taking in count there are 4 levels that boost production speed, I'd show the lowest level(the one that takes more time than the rest) and add a note saying that higher level factories may produce the unit faster)