Questions Opportunity and Sales Order Join - Missing Data

Hello forum.

My company recently launched Acumatica, and this forum has been extremely helpful.

I recently joined the opportunity and sales order tables (and vice versa) using the method described here:

https://www.augforums.com/forums/acumatica-generic-inquiries/opportunity-screen-relation-to-sales-order-number/#post-6401

It has been working perfectly until recently. I've noticed a behavior where the association isn't being made for some reason, which affects some other integrations I've setup with Zapier. If you can see the image below, the sales order ID is being pulled over for some opportunities, but not all. On the sales order screen, it looks like every sales order is being correctly associated with the correct opportunity ID, so the issue is isolated to opportunities. Also, these opportunities do not have multiple sales orders associated either.

Any suggestions on what could be going wrong here?