Not sure that leaving the game has any effect any more on one's performance in a competition.
There was a time when leaving the game after catching a target fish would land you another when you returned, or change your RNG, but FP said some time ago that this tactic had been addressed (fixed). I also regularly observe what you report but it doesn't bother me, as I also have paused a competition many times, since I have young children to tend to and regular life things to do during comp times, so a 30 min comp can take me 90 minutes to complete in such cases.
So far I have not seen any benefit when leaving or pausing during the comp, Just yesterday, in a South American competition, I observed a player who regularly does well, who was failing to produce their regular results. Multiple exits and re-entries over the course of the two hours did not increase their standing, so I am inclined to believe "no harm, no foul".