What client you on? It’s patchy on Yakkihone/Nostrudel/Nostur, even when I’ve muted someone on all three, which I’ve only done with one account and their mug still appears when they’re on their weekly day trip from X.
Discussion
Sometimes mutes between different clients don't work. The spec has the option for public or private mutes, and some clients support one or the other. I've noticed this will wreck my mute list sometimes, reminiscent of when your follow list gets deleted while using another client.