Note, the E-m1 mark II viewfinder uses a TFT LCD type display, similar to the display used in most cameras rear screen. The E-m1 mark I/III, E-m1x, E-m5 mark I/II, E-m10 mark I and Stylus-1 all use a similar screen.
The E-m10 mark II/III/IV, E-m5 mark III, and Pen-F all use a different type of screen (OLED).
Typically the rear screen of Olympus cameras uses a TFT LCD display. The only two cases where I know Olympus used an OLED display is the E-m5 mark I and Tough TG-2.
One 'feature' of TFT LCD screens is that if you wear polarized sunglasses, one orientation has distortions or is completely opaque while the other orientation is fine. For Olympus viewfinders (including the old VF-2/VF-3/VF-4) the problematical orientation is horizontal or landscape orientation. For the rear display, the problematical orientation is vertical or portrait orientation.
With the exception of the E-m5 mark II and the VF-2/VF-3/VF-4 external viewfinders, I tend to see the distortion in the viewfinder as being horizontal waves, where some parts of the screen are readable, and others are not. When I tested them, the E-m5 mark II and the VF-2 viewfinders were completely opaque when I used polarized sunglasses. I find in practice, if I use a single focus point in the area that is not distorted, I can get shots letting the camera autofocus. I would not be able to manually focus with the distortion (but I rarely use manual focusing anyway).
Due to photophobia being a major migraine trigger for me, I need to wear polarized sunglasses all of the time when I'm outdoors in daylight, so for me this is an important issue. Others just take off the sunglasses to shot.
As to why Olympus uses the TFT LCD viewfinders in the higher end (and older) cameras is that they can get a faster refresh rate with TFT LCD (120fps for the E-m1 mark II if memory serves compared to 60fps). When I tested my E-m1 mark II, I found that the camera used about 16% more power when I used the higher refresh rate. Note, there are other cameras (notably the Panasonic G9) that claims to have OLED displays with faster refresh rates. Olympus does not seem to use these types of displays.
I have talked to other migraine sufferers that didn't need the polarized sunglasses, but for them the faster refresh rates were important, and they liked the TFT LCD displays for that reason. I think in older cameras, Olympus used 30fps for the OLED displays and 60fps for the TFT LCD displays.
I find in general, that cameras with OLED viewfinders (or OLED rear displays like the E-m5 mark I) that the colors seem to be a little supersaturated. I mentally have to tone down the saturation when I'm trying to evaluate what the photo will look like, or if I liked the saturation, I would have to bump up the levels in post processing.
Back when I got my refurbished E-m5 mark I, I had the viewfinder fail on me when I went on a vacation to Disney World in 2014 (just t the 90 day mark where the normal warranty expired). In this case, the backlight on the display failed, which meant I could bearly read the display. Fortunately the camera was covered under the extended warranty I bought, and when I got home from the trip, I sent it in and got it repaired. It is still working as of 2021.
Perhaps in your E-m1 mark II the backlight has failed. I've seen another report of the E-m1 mark II viewfinder fail in this way, so perhaps there was a bad batch made. While I recently bought an E-m1 mark II, I haven't used it that much due to covid issues as well as other health issues that kept me house bound.
FWIW, OLED displays do not use a backlight, so they wouldn't see this type of issue. Also OLED displays tend to show true black better than TFT LCD displays.