I've had this experience for years. And years.

I think of it as "chipmunk syndrome".

Even the low notes on a mandolin are pretty high. But on a really good old Gibson or something, they growl in a quite acceptable manner.

The other thing about practicing mando is it's not the most comfortable for your fingers. It's hard to play quietly--the dynamic range is not there.

But it's fun and, hey, get a guitar in the room and you will be glad of the mando. Made for each other.

Willie-O