Subject line UTF-8 encoding

We have been happy with your plugin for years. But for the past 1-2 months, we have had problems with our subject lines showing, in some cases, complete ‘garbage’ full of question marks and, in other cases, html codes for special chars. We’ve tried turning on UTF-8 and turning it off but can’t find a setting that works for all subjects. Even more concerning is that in our small (3-person) web team, we are getting inconsistent results. So for example, two of us get all the question marks in a topic of no special characters with UTF-8 turned on, while the other gets a good subject line with UTF-8 turned on (even if it has special chars). Any recommendations? We’re staying current with releases on our test system so we’re on 2.11 with WordPress 5.5.1.