Then you clearly missed out all the drama concerning image retention with the very first Mac that came with a retina display. That was back in 2012, iMacs only get a retina display in q4of 2014 about 2,5 years later 😉 There are even people complaining about this very same issue on their iPhones.
Clearly this is not an iMac issue, it's a very generic one. The difference is in workloads and how good you are at spotting them. It happens in companies a lot where a lot of people are only working in 1 application and on displays used for presentations. Quality differences in TFT panels (even within batches) are a reason why one panel is more prone to this than the another even within the same batch. In some cases you can even clearly tell if the company has a "always lock your computer" policy by looking at the display. Usually the lock screen is burned into the panel. Rather funny to see 😀
Clearly this is not an iMac issue, it's a very generic one. The difference is in workloads and how good you are at spotting them. It happens in companies a lot where a lot of people are only working in 1 application and on displays used for presentations. Quality differences in TFT panels (even within batches) are a reason why one panel is more prone to this than the another even within the same batch. In some cases you can even clearly tell if the company has a "always lock your computer" policy by looking at the display. Usually the lock screen is burned into the panel. Rather funny to see 😀