Because of this issue, we too are looking for a different program to use.  If I find something useful, i will post here and ask if others who are looking to cancel this program and use another please post your thoughts here for the benefit of all.

This feature is essential for our large team that shares a single board that pulls mirrors from other boards. Without being able to filter by card assignee, our board has become unusable and our entire workflow destroyed. We will need to explore other tools if not resolved soon.

This feature is killer to me... Only figured out after migrating everything over to this structure.

When Card Mirroring came out, we thought it was a game changer as changes can be made on both cards.  We were very disappointed to see that mirrored cards do not show up when we filter by a label (in our case-the label 2025 and the board owner).  This now makes our whole process worthless and if there is not a fix soon, we will be forced to cancel our subscription and move to a new Project Management System.  Is there a timeline for this fix?

For us, filtering by members and the shortcut Q is a mega valuable addition. This is because we are now getting completely lost in the number of cards you do not need to do anything with yourself. Already very happy with the ability to mirror maps but the filtering feature makes it perfect. 

I created a workaround using board button automations. 

I copy all the cards of a board in a list named export, move all the export cards to an Overview board and assign the cards to overview lists depending on their labels. 

I can repeat this procedure with multiple boards creating a filter possibility. 

Afterwards, I archive all the cards on overview-board with a second board button.

We use Trello cards to organize our different departments tasks for IT and consolidate these cards from multiple boards in the development board and attach persons for responsibility for each card.
Before mirroring was introduced we copied the card and had to maintaine and synchronize these two cards.
Mirroring really helped to just work on one card, especially if requirements change or will be added.

But we are missing the possibility to filter the cards on the target board. Right now it is not possible to see all the tickets for e.g. one person which will help us to not overload the employee with too many cards. It would be greate if this can be added.

yes. I make frequent use of the 'q' shortcut to see cards assigned to me, but it doesn't include mirror cards assigned to me; so I can't use the mirror cards feature yet