Buffering even when not using

Andromeda59 wrote: "It does still have its original battery and I know it needs replacing (76%) but trying to eke it out for a bit longer.

Could that be the reason?"

Andromeda ~ Yes, but more than "Could be the reason", 76% maximum capacity almost certainly IS the reason. And don't try to "eke it out for a bit longer" – as the battery could become so weak as to brick your iPhone such that you can't even prepare it properly for battery replacement.

iPhone Battery & Power Repair

About genuine iPhone batteries

Repair Provider Verification

(Articles found by searching Apple's Knowledge Base.)

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If that happened to me, I would try these three things in this order:

1. Check Settings > Battery > Battery Usage by App and see if you notice any surprises.
2. Check Settings > Privacy & Security > Location Services and see if you notice any surprises there - apps recently tracking that you didn’t expect.
3. Close apps on your iPhone. Close an app on your iPhone or iPod touch - Apple Support

Andromeda ~ Does your iPhone 8 Plus still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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