Not sure what's going on in my case, but they don't get stunned when I shoot them in the head. They don't even flinch. I tried again just now and one guy completely ignored being shot in the face as he rushed and grabbed me.
The difficulty is way higher than it was in the original game, but mostly because the combat feels very inconsistent (which is by design, but I'm not a fan of that).
The original game had very tight and predictable mechanics - you always knew what animations/moves Leon and enemies will perform, the movement was very solid and gamey, and all of the enemies had predictable behaviors. The remake, on the other hand, is very "loose" - just like in RE2Remake, enemies tend to have inconsistent hit reactions (sometimes a headshot makes them flinch, sometimes it makes them to fall on the ground, sometimes it sets them up for a melee attack, and sometimes they just tank the damage and keep going).