Slack not recognising Screen Recording permissions

I have two users who are reporting that the Slack app continues to send them to the System Preferences→Security & Privacy→Privacy->Screen Recording list when they try and screen share. They have confirmed that it is checked. They have restarted the Mac, and tried re-installing Slack. Nothing seems to work. I asked one to go in to the Slack Preferences→Audio & Video and run Run an audio, video and screen sharing test and usually it says Screen sharing is denied, occasionally it says it should work, but it doesn't when they try again. Of course I cannot replicate the problem at all.