How much memory do you have installed on your Mini compared to your i7 2012 Mini? Ideally, if you have a 2012 Mini and maxed it out to 16Gb RAM and it gets warm, you should up your Mini 2018's RAM to double of that.
Part of the reason the Mini 2018 is getting much hotter near the top surface is that, it has an NVMe like storage SSD and when that gets used as real-time memory paging, as it can, to swap in and out memory, it will get super hot. The speed of the built-in SSD in the latest 2018 Mini is so fast that it practically act as physical RAM, doubling what you have now installed. Most NVMe blades need heatsinks so they don't thermal throttle. The 2012 Mini use normal SSD (if you have it installed), not a blade NVMe, so they don't get as super hot. Upgrading to more physical memory will lower the temperature of your Mini as it reduces memory swapping between storage SSD and physical RAM.