As described, the heat may be being generated by the magnetic inductive charging dock used to charge a second generator Apple Pencil. The charging dock is located centrally on the long edge of the iPad - on the side opposite to the physical volume buttons.
Unless you are charging a Pencil, this should not create any heat.
You might be well advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad:
Genius Bar Reservation and Apple Support Options - Apple
Assuming that your iPad was purchased directly from Apple, should you have any concerns whatsoever, you have benefit of 14-days within which to return your iPad to Apple for exchange or refund. If you suspect a fault, you would be well advised to this opportunity while you are able to do so.
Beyond 14-days, your iPad will be repaired or exchanged under Apple’s one-year limited warranty. Depending upon your country/region, you may have additional rights concerning available remedy.
iPad Service & Repair - Apple Support