Apple - Replacing rubber spring/cup under Tab key cap

These sort of surgeries are always "twitchy" at best. Worse, the new butterfly key design makes these way harder. With the original chicklet style keys, a good technician would get 50 to 90% success rate. I would expect the inverse - high failure rates on the 2015 and newer key designs.

The tolerances in the keyboard area (mostly the scissors) are very, very thin - and at some point, extra hard superglue or some cup that's 0.2 mm to the left of where it should be will require you to get a proper replacement.

In the pre-unibody days, the keyboards were a single piece that you could easily exchange.

Now, the keyboard is screwed into the unibody frame, so it's a very deep surgery to get everything removed and then handle more than 50 exceptionally small screws to get at the back sides of the pieces where the rubber springs that yield the touch for each key are mounted.

Lastly - even if you do replace things - the keyboard action is not what it was (and most likely even close) - so if you're the type of person that would keep on typing even if the key cap was missing entirely - go for a repeat repair on the rubber nubbin with a razor blade and superglue. If not, it's time for a proper repair or using an external keyboard while you save money for a replacement.

If you were to get a precision knife to clean out the prior repair and choose a more flexible adhesive, you might have better luck.

I have this issue as well, after forcefully removing the perfectly fine old rubber cup to install the one that came with the replacement. I have used double sided tape to install it, leaving a gap for the activator. Not sure how long this keeps working, but it does work in principle.