Hi Andy,
It’s not an issue I know of and isn’t something I’m immediately able to replicate, either.
Just to cover our bases: you are clicking Save Ticket after making this change? Also, are you in a position to perform our complete troubleshooting steps?
The basic idea would be to deactivate all other plugins and switch to a default theme such as Twenty Seventeen – then create a new event and set of tickets and see if the same thing happens under those conditions (this would help us to discover if the problem is the result of a plugin or theme conflict).
Of course, you may prefer to run through those steps on an isolated test site – which could be as simple as a fresh WP install in a subdirectory.
Last but not least, I’d additionally recommend that you update to the latest versions of all our plugins.
Thanks and let me know how you get on!