@broberg
I’m afraid I can’t add too many details.
The mirror I’ve used is this one:
https://www.glashandelonline.com/spiegels/confrontatiespiegel/confrontatiespiegel/
but it doesn’t indicate how much reflection/transmission it has (probably should’ve asked the shop first but I didn’t think about it untill I could test the mirror for the first time).
I also did a test with my phone behind the mirror, with its screen at maximum brightness and this gave pretty much the same effect so I think the problem is mainly due to the transmission of the mirror.