Are some Smart Keyboards not compatible with the iPad 8?

It should work, unless it's defective.

See also this page for compatibility:--> https://www.apple.com/shop/ipad/accessories/keyboards?page=1&f=ipad8gen2020&fh=35d5%2B4d0d

Take a look also at this support article:--> If you can't connect your iPad to your Magic Keyboard, Smart Keyboard, or Smart Keyboard Folio - Apple Support

Regards

Giulio

There is ONLY one Apple smart keyboard that works with the iPad 8th gen.

All other third party smart keyboards use wireless Bluetooth as their connection method.

Here is the ONLY Apple “smart” keyboard cover made for the iPad 7th/8th gen and iPad Air

Smart Keyboard for iPad (8th generation) - Apple

Apple smart keyboards for iPads use the magnetic, smart connector pins found along the bottom edge of older design iPad models and along the back of the iPads that use the 2018/2020 newer iPad designs.

What third party make of keyboard did you purchase for your iPad 8th gen?

You may need to connect it via Bluetooth.