Thank you to all who have replied.  Someone pointed out to me something I had overlooked completely, since I have been so focused on the absent "Reply to Sender" link in one of the two formats:  BOTH have an email link for the message's sender at the very end of the cluster of links, right after the Unsubscribe entry, and in both cases that mailto email link is enclosed with square brackets.

So until and unless Groups.io figures out why this is happening, the ability to privately respond to the sender of any given message still exists.

There is a difference between the Reply to Sender link and the mailto link at the end of the cluster, and that is that the Reply to Sender prepopulates the Subject of the email created with the prefix, "Private:," followed by the name of the topic.  This can, of course, be changed.  The mailto link just causes an email composition window to open with the To: field prepopulated for the sender, but nothing in the Subject, so you have to put whatever subject line it is you'd like.

Again, my appreciation for all the assistance that's been given.  Although the resolution is not (yet, anyway) the consistency I'd hoped for with a Reply to Sender option there is a viable workaround that's easy enough to use until the issue is resolved.
--

Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042  

[Regarding the Supreme Court refusing to hear the case brought by Texas to overturn the votes certified by 4 states:]  Pleased with the SCOTUS ruling, but also immediately slightly terrified of where this crazy train goes next.  We should know by now there’s a bottomless supply of crazy.

        ~ Brendan Buck, former adviser to Speakers of the House Paul Ryan and John Boehner