External Monitor attached to 24” iMac shows Input Not Supported when woken from sleep.

I have an Acer 24" monitor attached via thunderbolt to HDMI adaptor and cable to my 24" M1 iMac running OS 12.2.1. When woken from sleep the iMac is lightning fast using touch ID but the Acer monitor wakes with a black screen and a blue message box saying "Input Not Supported" slowly stepping diagonally down from the middle of the screen. This stops and is briefly replaced with a coloured static screen which is then replaced by a series of rolling screens made up of multiple panels that contain some of the image that should be displayed there. Each rolling panel display only lasts a couple of seconds and is replaced, after a brief black pause, with another in which the panels get larger. Eventually, after about 40 seconds, the screen displays as it should and is fine (until woken from extended sleep again!). I am guessing that the iMac is trying to discover what the correct refresh rate is for the screen or vice-a-versa during this waking up process. Eventually it works it out and all is good.

I am hoping someone can tell me that there is some setting I can adjust that eliminates this "getting to know you handshake" period and allows the 2 screens to wake up simultaneously.

Note: I used to have the Acer monitor attached to an old 24" iMac (2009?) via an Apple HDMI adaptor cable and had no issues with it at all.