Basic question regarding accessing previous imported images

The reason for dj_paige’s answer is that, by default, immediately after importing Lightroom Classic switches the Catalog panel to view Previous Import, a built-in smart collection. Naturally, when you import Batch B, that fulfills the Previous Import criteria and Batch A no longer does.

The key is to know that you’re looking at Previous Import which is a dynamic collection (it changes as file criteria change), so to find any images imported earlier you need to look in either of two places: You can click All Photographs to see all images managed by the currently open catalog. Or, if you know the name of the folder where the file was imported to, you can find that in the Folders panel which also contains all images managed by the current catalog.

On the left hand side of the Library Module, click on "Catalog" and under that click on "All Photographs"

Or if you imported these into two different folders, you can go to the Folder Panel on the left and find the folder that Batch A is in.