It’s hard to know what terms OP used correctly, but sounds like maybe the bath spigot is leaking a bit while the shower is on. If that’s the case, it wouldn’t cause damage to not fix, the shower water pressure would just suffer. And AFAIK, you wouldn’t typically need access to inside the wall, unless they did something stupid and tiled in a way that you can’t access the diverter stem nut to replace it.

IANAP, just DIY and have had to replace shower stems due to failing gaskets