If this is a design flaw, it's a massive one. If it's a bug, it should be a priority considering the serious cancer this community suffers from.

When I mute a person who suffers from a severe IQ deficiency and wishes to cope with it by constantly yelling utter nonsense in their mic, effectively preventing me from hearing footsteps while decreasing everyone's desire to see people below a certain IQ limit be processed into fuel; I would REALLY, EXTREMELY appreciate if my mutes remained instead of being deactivated a few minutes later for completely unknown and illogical reasons.

If I mute someone for very obvious reasons, I do not wish to hear them 5 minutes later.

Agreed. If they wont let us kick at least let us mute them.

I think this has to do with the respawn and waves. it seems that if the person you muted dies and then respawns, the mute selection resets back to default.
I haven't had a chance to confirm this but it might be what is happening.