I would probably want to report that as a bug if I had been using Discourse for longer. It makes the change on the basis that the previous behaviour is undesirable, removing behaviour that admin and regular users will have come to expect and may or may not desire.
In addition, the way the feature is described implies that the purpose is to give admin more control over the order, however that feels incomplete as there is no means of reordering the pins besides unpinning/pinning.
I think pin ordering should be a setting at category level. An option to choose between bumped_at and pinned_at, defaulted to bumped_at for a relatively simple change that doesn’t affect existing admin. (I recognise that is made non-trivial by localisation though.)
Longer term, I think this should be a fixed pin positions checkbox with UI to reorder the pins much like the UI for reordering categories when using fixed category positions.