As observed over on TDWTF:
Notice the list, which was saved.
And now:
The discussion revolves around a bug in the “Civilised Mute” feature where the muted list does not persist after saving. loopback0 shares a screenshot of the issue and tries to reproduce it. sam asks if the issue can be reproduced and later suspects that another profile edit might be causing the issue. However, loopback0 confirms that no changes were made to the profile after saving the muted list. sam investigates the issue and suspects cross-user contamination in the delete statement. After further investigation, sam confirms the bug and later fixes it, committing the fix to GitHub (commit).
curious can you repro this here?
Let’s see. Might take a couple of days or views or whatever.
Here it is set now, using users picked at random from the topic list. And Nagesh.

Maybe some other edit to the profile causes this ?
So this was after the first reload which I did straight away. It definitely saved it.

A couple (maybe three) more reloads later, spread about a minute part.

Gone.
No changes to the profile made since saving the Muted list.
wow … something crazy is going on … having a look,
my gut is telling me there is some cross user contamination for the delete statement.
I can see this bug as plain as mud, will get it fixed shortly
WOW I should get an award for such a bug
https://github.com/discourse/discourse/commit/14741b5dacb4774893908aed713503cdf1b94960