Why would there be both stranded and solid conductors in the same panel?

Some conduit and some not?

why would be it be a problem?

Not worried about it just curious as to why.
It’s all in conduit.

Probably it was done by different contractors , Some carry strand in rolls . and it is easier to pull through conduit.

The solid was probably originally put in by a resi. The stranded was probably put in later by an electrician who is used to commercial work and prefers stranded over solid.