imo, it's because you're gauging your playing with them on their activity with others, not on your activity with them. you're seeing them play with someone else and deciding to slow-tag or backburner them because of the sole reason of their activity elsewhere. it's not how they tag you. it's not that they're a shitty tagger. it's not that they're doing anything to you other than not just tagging you. going by what you said, which was that you're taking it as them shopping around so you're going to react according to that fact in your head.

your rp partner is just playing. you're the one monitoring and reacting to their activity outside of their playing with you. that's weird.