I never noticed this as I only usually display the time, but if I update the Clock options to display both date and time, it's showing one on top of the other as well.
This is on Debian 12 with XFCE, though, but I found a way to fix this by using a custom format, taken from XFCE's panel documentation. You have to select "Custom Format" describing how you want things to show up:
The linked documentation describes what each of these symbols mean.