Hi James,
I’m sorry that you’re running into this issue with recurring events – there was a known issue where “Once” rules were not being saved, which was addressed in the latest release. Can you first make sure that you’re running the current versions of both The Events Calendar (4.6.23) and Events Calendar Pro (4.4.32)? Make sure to clear out any caching that you might be doing on your site or via a plugin after updating; you’ll also want to temporarily disable the “Enable the Month View Cache” setting under Events > Settings > Display, if you currently have that enabled.
If that doesn’t get things working again, can you please share your full system info with me, along with a screenshot of the settings that you have for this recurring event (with all recurrence rules expanded) so that I can test it out on my end?
Thanks,
Jennifer