Updating repeat params doesn’t always work

Trying to make an event repeat on a particular day of the week, I find I have to Update three or four times before the changes will actually stick. I also find I have to go back and forth between “Week” and “Advanced” until one of them works. It’s an intermittent problem, so I can’t give you steps to reproduce beyond that.

I’m on an iMac Pro running Firefox/Mojave. Thanks!