Solution
Different Resolution monitors cause unreal to crash. Either stop using the different resolution monitor or artificially change the resolution of the monitor using software (Nvidia have a software for GeForce cards to change the resolution of the monitor, but the issue is they warn it voids monitor warranties so I chose not to use the monitor).
Explanation of Theory
The input HDMI and the Display Port does not seem to be the issue. The Wacom runs off display port and the Samsung runs off HDMI but moving UE windows between the two does not cause a crash.
The refresh is also not the issue. I set the Alienware down to 60Hz in line with the other two monitors and it did not fix the issue, the editor still crashed and crashed my whole PC with it when moving editor windows.
The only other difference between the monitors is resolution. The Wacom and the Samsung are both 4K screens (3840 x 2160), the Alienware is the one which is (1920 x 1080) and this is the one where if I drag a window onto it then it crashes. So, my best theory is the resolution difference is the issue. What does confuse me though is the fact that I am sure I used the Alienware as a second monitor for Unreal without this being an issue back at the beginning of January 2021 but now every time it just crashes regardless of the engine version. So, if you know more about this than me please get in touch by leaving a comment! Thanks