Chris,
I did several tests including creating a new gift certificate simple product and I was able to throw an error that as I type this out may have just given me an idea. I believe the issue is due to db fields not set properly for Joomla 4/5 settings.
When a site's been properly updated to Joomla four or five, these database updates are usually done correctly. However, what may have slipped through the cracks is when a plug-in adds additional fields or database tables where the installation of that plug-in wasn't updated to adjust those new database fields properly, which is what I think is going on here since the gift certificate plug-in does add a new database table.
Is this the latest version of the gift certificate extension or is this the first one that we sent you the link to download when it came out?
I believe in the latest update it did account for those database changes. So if you could download the latest version, install it and see if that fixes it it would be great. Otherwise I will take a backup of this so I can look at the database tables myself and make sure that they are all set properly.
Adam