How can I stop sound from going through on muted iPhone calls?

I often call with my partner while they're asleep, so I mute my microphone as to not disturb them, but after a while audio on my end will start picking up again even though it says i'm still muted. I have the iPhone 15 plus, it shows the audio waves from either end of the call on the dynamic island at the top of my screen, it shows audio waves on my end while still being muted after a certain amount of time, I know the sound is going through as well because my partner has told me. Has anyone else had issues with this? I've had this ever since I got this phone back in January and they haven't patched it, I can't see anyone else having this issue either.

[Re-Titled by Moderator]