My previous versions of Premiere Pro aren't available on Creative Cloud

As much as it pains me, this is not a bug.  I mean, I think it should be but this is intentional on Adobe's part.  Adobe only makes the preceeding 2 versions available of Premiere at any given time.  They institiuted this policy a few years back, and although they didn't say it, the consensus I understood was that it had to do with their discontinuation of a licensing agreement with Dolby (Dolby digital codecs used to be available for export in CC 2017 and earlier).  But they decided to make it a policy going forward.  So as of today, no versions earlier than 2023 will presumably be available.  I suggest you do what I do and make backup copies of your previous installed versions in case you ever need them, becuase it is not easy to get a hold of older versions later, unless, as I understand it, you're an enterprise customer.