Why is my iPhone 7 Plus unstable only when battery charge is below 30 percent?

I use iOS 15.6.1 and since I got a service warning about Battery Health being 79% –saying the battery is degraded and should be replaced – the iPhone 7 Plus behaves erratically when battery percentage is below 25-30%.

When fully charged it works flawlessly. I’ve been able to replicate this problem several times.

Is it because the battery is giving off bad voltages or cannot supply enough power for iOS to work properly when it is below 30% charge?

It’s very unstable when battery is 25-30% or less.

Symptoms are it launches multiple apps by itself and types on the screen without me touching it. Touchscreen becomes less responsive and difficult to enter passcode.

If above 30% charge it never happens. Works absolutely perfect and stable above 30%.

I’ve done a hard reset and reset all settings to factory defaults to no avail.

Will continuing to use the iPhone with a degraded battery damage the internal components of the device? I can’t afford battery replacement yet.

Thanks in advance for any insight or advice about this.