Came across a weird file/result that co-workers were able to replicate.

1) Mojave 10.14.1 and Illustrator 23.0.1
2) Selecting two shapes/pathfinder window/unite
3) Letter p remaining knocked out
4) Letter p featuring a slash across the top
5) I've attached the file before uniting with pathfinder

I was able to workaround by simply not uniting but thought ya'll might be interested. Thanks!