Hag

This is an obvious issue. If you set a Repeating Command Block to always active, it will execute the command on each tick.
You can't really do anything about it, perhaps extend your RAM.

You mean, these letters?

On a sidenote, I set my Switch to Japanese just because of this one and I wanna thank you for reminding me how cool the Japanese cover image of BotW is.

I suppose you are both connected to the Internet. Try inviting device #2 to device #1's game.

If the world is generated using the Default (not sure about Old), you have to look for the End Portal using Eyes of Ender.
If you are in a Flat world, make sure all the portal blocks are facing inwards, like the screenshot I uploaded.

I think this is a common issue. Getting pushed around by pistons continuously is no good. This could also be the result of the relatively low memory (RAM) of the Switch.
I still think the first one is the possible cause.

I can confirm this issue, it's happened to me countless times. Not all your hits directly on the drowned are registered.
At first, I thought it was the slightly bad internet connection that was the cause. So, I disabled the Multiplayer Game option, and after finding yet another drowned, it occured again. Before this happens, the drowned seems to shake / vibrate very lightly for a fraction of a second.

I managed to reproduce this on Nintendo Switch and on mobile too.

02.01.2020.: I uploaded a video file to the issue showcasing the bug in action.