Just making a note here in case anyone else runs into this problem.
Yesterday (Nov 1, 2018), I noticed that I could not properly log into our web server (joomla) CMS. The problem was that some redirects in an htaccess file had stopped working. I assumed the problem was due to a new joomla update, and ended up spending many HOURS trying to add new mod_rewrite rules in htaccess to work around the problem. It took hours because I could not convince myself that mod_rewrite was not working correctly. I just assumed I was making mistakes in the mod_rewrite htaccess rules, which has happened so many times since it's so finicky and obtuse.
Long story short, after about 10 hours of banging my head against the wall trying to understand why some htaccess rules were not behaving as I thought they should, and experimenting with different rules (and breaking the site continuously as I did), I tested the rules that weren't working on another web server, and they worked just fine. That's when I realized that something was wrong with mod_rewrite. Checking our server control panel (cpanel) I saw that apache had recently been updated to v2.4.37 only a day or so ago. There was no more recent version and no reported bugs of the nature I was encountering, but I did notice that there were some related apache helper files that the cpanel EasyApache setup was willing to update. After doing that everything was working again.
So this is just a heads up, and maybe especially relevant for others who have cpanel and apache -- if you are seeing misbehavior with your htaccess rewrite rules, check into whether a recent apache update (or EasyApache on cpanel) isn't the cause.