Its auto dimming. Something must be wrong with it. Actually the first time I noticed it I thought it was an unmarked police car behind me. I don't have any aftermarket tint on the rear window so that's not it.
I've noticed this effect on two of the four cars I've owned with auto-dimming mirrors. unno:
Hopefully, 'You'll get used to it".
If it annoys you too much, bring it to the dealer (after dark). Ask to drive another new CR-V with that mirror. If that car is better, ask them to fix it (probably would required to replace the mirror).
PS: Glare or halos around light sources at night, can be the first indication of cataracts. :geezer: