Situation:

Added in a bunch of games through import from Steam.
Completionator auto-set the Aquisition date as the import date.

I individually update a games Aquisition date. I go into my profile and click sort by Aquisition Date, the game is still set by Steam import date.

I individually click on a game, the manually added aqusition date is still there saved and fine.

Does it just take some time for it to update on the profile end? Or is there a bug...?

I think the site is working properly, but I think the UI is a little confusing in this scenario.

There are two distinct dates on every collection entry: Added On and Acquisition Date. Added On is simply the date / time on which you added the item to your collection, doesn't matter if it was a manual entry or through an import. This date cannot be edited and is never updated.

Acquisition Date, as you've already found, is an editable field on each collection entry. By default, it's empty, but you can enter a value (as you did).

On the Collection page, there is a date that's prominently displayed in the gray-ish title bar for each entry, along with the item name, platform, etc. Although it doesn't have a label, this is actually the Added On date. If you look inside the lighter area of a collection entry, you should see a column that starts with Region. If you look at the bottom there, you should see a "Date" label. That value is actually the Acquisition Date. If you haven't entered one (or if you removed it), you should just see dashes.

When you sort by Acquisition Date, if you only look at the more prominent date (Added On), it can definitely look like it's out of order (because it's searching on the other date). I sort of confused myself for a second when I was recreating your scenario and I built the page

I'll have to think on this one to try and come up with a way to make it a little more intuitive.