I strongly doubt that this will be implemented, and here’s why: each record in a database is a unique and independent entity, and therefore can only be in one place at a given time. This is a core database paradigm that’s not likely to change any time soon.
The analogy I like to give here is that a record is like a person. Say that you’re a member of two different clubs, and both clubs take an annual membership photo. It just so happens that both clubs are taking their respective photos on the same day, at the same time. Because of this, you have to choose which photo you want to appear in. You can’t clone yourself and be in both photos at once. You have to choose to be part of one photo, and absent from the other.
Database records are the same way. When choosing to group all records at once by a given field that has multiple options, Airtable has to decide where to place each record. While technically it could “clone” the record as needed for each group simply for display purposes, it doesn’t because that’s not an accurate representation of the data, and accurate data representation is hugely important in database software.
To use your example, there aren’t multiple copies of Record 3. There’s only one Record 3, so it can only appear in one group. To see the full lists of both Group A and Group B, you’ll have to create a separate view for each group. Their full membership can’t be viewed at the same time, just as you can’t be in two photos being taken at the exact same time.