@jsailor Thanks, although I am still not able to view prior submissions and the prior submission comments, I have also not determined what exactly is happening and why. I do not have reproduction steps at this point. I just presume that this will resolve at some point. I only know that in the switch of this institution from a Singapore server to an Oregon server we went from Student Enhancements disabled by default to Student Enhancements enabled by default. We may be in a unique situation and have unique regression as a result. My impression is that few, if any, institutions, change servers. At the moment my plan is to leave the Student Enhancements setting enabled in admin until summer term ends, and if the issue remains, switch the setting off between terms and see what impact that has on fall term.
One possibility that perhaps I am misunderstanding: If I have allowed multiple submissions of a homework, do I have to click on the Reassign button to enable retention of earlier submissions? In the past I did not have to do anything in terms of "reassigning" but perhaps that is now the workflow?
Post-script edit: And today I was unable to reproduce the issue when working on a different computer (both Mac OSX Big Sur 11.4 using FireFox). As I said, I also cannot consistently reproduce the issue and I presume that it will eventually go away...🙂
Further edit: 
As of June 24 I am still seeing instances where there ought to be drop down list access to a prior submission, but there is not drop down list. See above, this is a resubmission as per the screenshot, but there is no drop down list access. This is on FireFox 89.0.1, Mac 11.04, but my gut sense is that this is possibly related to students probably submitting via the Google LTI 1.3 submission method for a file "upload" from a Google Workspace for Education account. With classes asynchronous and online, I cannot verify this is the issue, but if so, then the issue is an external tool interface issue. I am working to be more careful to click on the "Reassign Assignment" button in the hopes that this will trigger retention of the earlier submissions.