I found this post from October 2021 (as it happens, it was a post of mine, but the content was copied and pasted from the Apple support page for Monterey):
- Available on MacBook Pro (2018 and later), MacBook Air (2018 and later), iMac (2019 and later), iMac Pro (2017), Mac mini (2020 and later), Mac Pro (2019), iPhone 7 and later, iPad Pro (2nd generation and later), iPad Air (3rd generation and later), iPad (6th generation and later), and iPad mini (5th generation and later). Older iPhone, iPad, and Mac models may share content at a lower resolution to supported Mac models when “Allow AirPlay for” is set to “Everyone” or “Anyone on the same network” in Sharing preferences.
Now clearly this was before 12.3, and it says Mac Mini (2020 or later).
So I can only ascribe it to luck that the 2018 mini worked for a while, but it was never a supported configuration.
The post was in this thread: What are the requirements to have AirPlay… - Apple Community