So here's the steps to reproduce:
1) Host machine is set to 1080p.
2) VM settings is set to "use host display settings/resolution" (the recommended setting).
3) Start the VM, put it in normal window mode (not full screen, and not unity, just the resizeable window), and set guest windows to 720p resolution.
4) Now enter Unity mode, then exit Unity mode.
5) Observe that the resolution should return to 720p, windowed mode, as expected.
6) Enter unity mode again.
7) Now, start a video game, and tell the video game to go into "full screen at 1080p" resolution.
8) Unity mode streches to fit the whole host screen, as expected.
9) Now exit the video game. You are back on the desktop, which is at 720p resolution.
10) Now exit unity mode.
11) Observe that exiting unity mode has now forced windows to quit being 720p, and changed the resolution to 1080p.
Sorry for the long repo steps, but the last step is where the issue appears. If you are in Unity, and you run an application that temporarily changes the resolution from 720p to 1080p, then Unity mode will accomidate that change, but then when you exit that program and return to windows desktop, the resolution of windows desktop is 720p, but unity mode doesn't recognize that anymore and as soon as you leave unity mode it will change windows' resolution to whatever was the biggest resolution that it experienced (aka the higher 1080p resolution that occured while the game was running).
The root of the problem seems to be that unity doesn't seem to recognize the resolution of the windows desktop. It only respects the resolution of whatever app or game you run (and many games do change the resolution, especially if they are running in the game's "full screen" mode). Unity seems to recognize the game's resolution, and makes it work on the host, no problem. But then after the game is closed, and you return to the desktop, unity mode doesn't seem to notice that the resolution went back to whatever is the desktop's resolution. And so if you then exit unity, it will put the resoltuion to the higher resoltuion that the game previously was running at, and you lose the resoltuion you set for your desktop.
I like running my desktop at 720 because it lets me keep VMWare running on the side of my screen without hogging my whole host screen. But then when I run a game, I have it at 1080p so it fills my whole host screen. After the game is over, I exit unity mode, and it's changed the desktop resolution to 1080p.
But again, this problem doesn't occur if I don't actually play a game. For example, if I'm on the desktop, and I simply enter unity and then exit unity, it goes back to 720p. It only changes the desktop's resolution if I actually enter a game while I'm in unity mode, and the game temporarily changes the resolution... then it's messed up, and when I exit unity, it'll change the desktop resoltuion to whatever the resolution of the game was.
Hope I explained this issue thoroughly enough.