Good evening Brandon,
Welcome to the Tribe and thank you for reaching out to us.
You are absolutely right. Your issue is very similar to the ones you have found.
Even the temporary solution you came up with suggests that your custom theme limits the number of posts returned in anĀ archive template.
The first thing I would do is to look for a setting in your custom Theme for the number of posts returned in an archive (if there is one).
If you can’t find anything, then you’ll have to look at the functions.php file of your custom theme and perhaps at the archive.php file too.
You will need to hunt down where this value is defined and tweak it.
To get you started, I recommend a quick Google search with the following keywords “number of posts archive wordpress”.
Hopefully, this is helpful to you and let me know what you find out.
Best regards,
Geoff B.