The Czech Senate (upper chamber of the parliament) uses top-two runoff too and a lizard-lizard competition in the second round happened in 14 out of 27 districts, which is only slightly more than half (lizard defined as a candidate of one of the two strongest parties). Remarkably, there was no lizard-free second round, at least one lizard succeeded everywhere. In the second round, 20 lizards won. It means that out of 13 lizard vs. non-lizard competitions, 6 were won by lizards. It seems to indicate that lizard victories aren't largely due to "strategic" voting: if it were so, non-lizards would massively outcompete lizards in a direct confrontation.

though others might appear, for example regional parties like there are in Canada

What reason for appearance of regional parties is present in a top-two runoff system and absent in the plurality system?