I think this is different from the daily first kisses that takes away the exhaustion, since that only shows a small red heart. This confused me when I first saw it too, no one really talks about it, you're the first one I saw talk about this lol
This one, heart speech bubble happens in random days for me too with Sam. When I kiss Sam randomly in the middle of the day, he sometimes react heart speech bubble (like that) or smiley speech bubble. There's also this adorable jumpy thing Sam does when I try to interact with him horizontally instead of vertically and fast-pressing the talk button until he jumps and have an exclamation mark speech bubble.
Dunno if that's a new 1.6 thing, but I never seen that with my 1.5 Sam. Also made sense that it doesn't show up as much, 'cuz I think few people (including me) actually interact with their spouse midday or after that first kiss of the day lol
Either way, these little reactions had made me come home midday to interact with Sam just to see if he'll do the jumpy thing, it's really adorable, and I didn't question it lol You could try to get more reactions out of your Seb if you want, I think all spouse can do the jumpy thing and reactions.