Pound cakes

I'm mainly going to bump your topic up but my guess would be to look at the conditions. E. g. is a different kind of flour? Is the pan bent? Is the oven not heating evenly?

Simple enough: try another pan.

If no change, try a different oven.

Then if no change, Try new baking powder or baking soda.

When that happened to me, it was because I opened the oven too many times or for too long.

The reason for the collapsing pound could be a problem with the brexit.

Leave the article 50 seconds longer or more before turning it out in the cold.

I'm assuming your recipe is tried and true.
It's not the pan, it's most likely the leavener. Is your baking powder old? Did you use a different kind of baking soda? Eggs? - Are you getting them from somewhere new? Flour and/or sugar, new or old? Same brand as usual?
Oven temp normal?