Your iPad Pro M5 is unlikely to have a fault.
Your iPad Pro M5 has features and capabilities (such as Apple Intelligence) that are absent from your much older and significantly less capable iPad9. Notwithstanding key performance and functional differences, the iPad Pro M5 dual layer OLED screen being considerably larger and likely using more power than the much smaller conventional LCD display of the iPad9.
Unless both iPads are simultaneously running the same Apps and processes, direct comparison of relative battery performance is at best unreliable.
If you have any concerns as to the health or performance of your new iPad, or suspect a fault, you would be best advised to contact Apple Support directly - and request a remote diagnostic test of your iPad. You can contact the Support Team using the Support link at top-right or bottom-left of this page.
Alternatively you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to hands-on test and assess your iPad. In the unlikely event of a fault being identified, you'll be in the best possible place to explore your available options:
Genius Bar Reservation and Apple Support Options - Apple