Mixing 3800/2800 APs with 9120 APs on 9800 controllers?

Don't mix x800-series APs with Catalyst APs.  

x800-series have a MARVAL radio chipset and Cisco has been trying hard to "tame" the bugs associate with them.  I believe they have already "given up" trying to fix these bugs on AireOS. 

https://community.cisco.com/t5/wireless/mixing-3800-2800-aps-with-9120-aps-on-9800-controllers/td-p/4748781

Mixing access points of different models and types

The Cisco Catalyst 9130 Series are very advanced access points supporting Wi-Fi 6 features along with unique features such as dual 5 GHz and advanced RF detection using Cisco custom RF ASIC silicon.

For this reason, it is not recommended that you mix access point models, sometimes referred to as a “salt and pepper” approach, as the 9130 Series can make spectrum decisions, such as dual DFS detection, that other access points do not partake in.

For this reason, if you have a mixture of AP types, it is recommended that you group like access points together (for example, Aironet 3800 Series on one floor and Cisco Catalyst 9130 Series on another) and refrain from mixing them.

https://www.cisco.com/c/en/us/products/collateral/wireless/catalyst-9100ax-access-points/deployment-guide-c07-743490.html