Thanks for your response. The doors have remained as 'Unstyled' and how you've described the Flip option only appearing once the door is inserted into a wall does apply to us as well. I've realised that on one project this is happening to was actually created in 2022, so can't be a Batch Convert issue.
We've tried the Update Plugins procedure you've suggested, thank you. Fingers crossed this is the solution - it seems to be working so far, but because it was an intermittent problem I'm not sure if it's solved it or not. Time will tell!