Due to the amount of times they've died and been revived, Ash randomly dies for about a day per region. Pikachu for less time, TR & Brock for even less, and other traveling companions for an even smaller amount of time. This is so their bodies can catch up with the fact that they've died and they often act a bit loopy for a few minutes prior. Pikachu honestly probably stays in Ash's arms for the whole 24 hours though, which is fair since he's probably dead for most of that time anyways.

>In Unova, it probably happened while the gang stayed with Cynthia.

>I imagine during XY/Z, it happens like the day after the Kalos Crisis, making everyone extremely worried. However, the Champions know what's happening.

>In SM, I imagine it happens while the gang's in Kanto. Ash just collapses while they're looking at water Pokemon or something and Brock starts acting loopy, so Misty has to drag Brock to the main lab building while getting one of Ash's Pokemon to carry him (and by extension Pikachu) to said building. The Alolans just get to be confused.