Custom Field: Modify how the value is shown in the "Estimation Bubble" of a Scrum Board


I am currently working on my own CustomField.
I got a bit into it, but face troubles when it comes to define how the value is shown in the small grey “Estimation-bubble” in the bottom left-hand corner of the card.

I already tried things like overriding the getValueforIssue method of my CustomField which extends from NumberCFType.
But that method is ignored here. It seems that it accesses the value directly from the database.

Could you explain here how this works, and how i can edit what the value in the bubble will be or how it will be displayed?

Thanks, Franz