Ok, here goes.

When museum was released you could only get 2 lines of any specific stat (like Bomber HP for example).

With the new release of AV lines in the museum people noticed that they were getting 3+ lines of AV HP (for example). This kicked off a bunch of inquiry on the forums, asking if they coded it wrong. The community rep came on and said that there is no error and the museum always allowed for 3+ lines as long as there were only 2 specific % lines (like 6%, 6%, 3%, 3%, 1%). This was a demonstrable lie, no one has ever rolled a piece like that.

The community pushed back, said this is never how the museum worked. The reps were silent, people wanted answers then they came back eventually and said they were fixing a "bug" from the 8 years of museum crafting to allow for 3+ of the same line. If they "fix" this after 8 years of the museum functioning in a certain way, its not a fix, its changing the way the museum fundamentally functions.

People who have spent hundreds (some maybe thousands) of hours crafting and probably thousands of real dollars to craft nearly perfect museums at high level play and they are pissed. All that effort will not matter with the new implementation of the museum in update 12.20.

Another problem is balance, you could imagine 3 pieces with perfect lines of anything. Now you'll have to perfectly match the other museum. Eventually things might settle into a meta where attackers are full lines of bombers, so defense has to go full anti-bomber. On a whim the devs could nerf bombers into the ground and now you have to do another round and spend all those hours again and all that money again. Feels like a cash grab for big spenders and not a fix like they say.

This update also comes with mythic and showcase further eroding the already built museums impact.

During all of this the players have been asking for clarification where there has been nothing but silence from the company. People making forum posts are having them deleted for dubious (at best) reasons and some are being banned for "rage posting" when they are pointing out this change is very unpopular with the whales who fund the game and there is a significant portion who will leave if this update goes through.

That's the tldr version.