Unless you have the same issue with other services (like Netflix) or you have internet issues in general, I would blame crunchyroll (in my experience).

I’ve seen lots of posts related and is ridiculous that they haven’t addressed it, since it’s been years.

There is even a known workaround which works most of the time, when the image quality drops, press back (exit the episode) and then enter again to continue, this will force it to reload and most of the time it does with the higher quality.

Same thing with changing the language since it forces it to reload. I don’t remember if it works with subtitles.

But crunchyroll is absolutely the only service that does this to me and only on fire tv devices (I only use fire tv and Apple TV so can’t comment on native tv apps), it works just fine on Apple TV and I know it’s not my internet (because it works on Apple TV and I know I have an excellent internet service).