By the way, I have dark mode on, but on apps that I use the most, like rif and Chrome I force light theme, as it hurts my eyes reading white text on black background. Also, I have brightness on manual. During daytime I have it at arround 80 percent and arround 40 in nightime.
Also keep in mind it is summer period for a lot of countries now. Meaning more time outside, more sunlight. Zenfone 8 has HBM up to 800 nits which triggers for outdoor sunlight readability. This consumes more power of course. (And not different from other brands with bright screens).
It might not be discernible as "brighter" because if you are outside, the ambient brightness is also very high - but it will be noticed in terms of display power draw. HBM can trigger in Auto and in Manual (when brightness is at max).