[throwawayosiu1]

We are paying salary as a perk. Legally we don't have to.

With that in mind, why should those 16 months count to experience?

Work was not done by said engineer, and experience was not gained.

The other engineer worked, gained experience that's valuable to the company -> hence the raise + promotion.

[meheleventyone]

Well you don't have to, it's a perk just as you don't have to pay their salary. Which is why the distinction is pretty arbitrary. And experience isn't the only reason to keep up pay level for example you want to remain competitive with the local market and inflation. Presumably you also value this engineer and want them to come back to the company rather than find a new one with better pay if they feel they are falling behind whilst on leave.

You could also make an argument about the gain in experience due to motherhood and being able to take an extended break from work. I believe I'm a stronger developer for having two children for example.