Dual Monitor Setup with Mac Mini (2014) Screen Blinking
I believe it to be a bug: it's a widely reported problem on a variety of Minis using HDMI, and I've certainly encountered it on my 2018 Mini.
A 'quality' HDMI cable may improve things. Turning off the 2nd display and turning it back on again may also make it snap out of it.
You don't say what OS you're running, but I would make sure that you keep up to date with OS updates, in case a fix is released. I believe that Big Sur 11.4 is expected imminently...