Thanks for passing that along.
I probably would have dismissed that if I previously saw it because being nearly 3 years old is much older then when my issue popped up last year when I upgraded from 5.3. However, at this point I’ll definitely give it a second look.
I’m currently experimenting with the Unfiltered MU plugin as a replacement for the code I posted above. I was initially turned off by the fact that it hasn’t been updated in 3 years either. I was swayed to try it from other comments talking about the fact that it’s written by the Automattic team and just because it’s old doesn’t necessarily mean it’s any less effective.
If I don’t have luck with the plugin after testing a while I’ll give what you sent a try.