The answer as to why the passenger side does not go dark is that, per your owner's manual, "The vehicle has an automatic dimming outside mirror on the driver side. The mirror will adjust for the glare of headlamps behind you." That feature is not available on the passenger side mirror.
I'm not sure where the mirror gets it's command from to go into dimming mode. Maybe it's using the same signal from the dash top sensor that switches the instrument panel and infotainment screen to "night mode". Someone with a schematic of the mirror electronics may be able to answer the question as to why the driver side mirror is dark during the daytime.
What I did notice, on my '18 Terrain, is that when shining a bright light inside the blind spot warning indicators on the driver side the "sparklies" appear in a grid pattern, while the "sparklies" on the passenger side appear in a random pattern. What that exactly means, I don't know. But it could inidcate that there's a photo sensitive array under the top layer on the driver side, which in your case is somehow getting some wrong data.
Did the mirror go dark during the day before you installed the convex button mirror?
2018 Terrain Denali, 2.0T, Blue Steel Metalic/Black
2013 ATS Premium, 3.6 V6, White Diamond Tri-coat/Brownstone
1967 Camaro, 327, Butternut Yellow/Black