So sorry for the delay, I somehow missed your request here! If this is still occurring, are you able to provide a link where I can test this out to try and reproduce it?
Offhand it sounds like it might be a caching issue. Do you know if you are using a caching plugin and if so which one? The Sign-up Sheets plugin supports a number of caching plugins so when a sign-up occurs it will automatically clear out the cached version of the page However, it’s possible the one you are using isn’t supported yet and if not I’d like to try and fix that.