I think you are right! It happens when I scroll down the page using my right thumb, which is in line with the like button. It’s odd, as it happens during the scrolling process (as well as when I deliberately click the heart). The software must react more quickly than me to what my own thumb is doing.
I now prefer the default Discourse behaviour, which greys out the topic and overlays a “Welcome back!” login modal. Less confusing to tired people like me today 
And the Guest Gate theme component is good at what it does too.