Google updated Business links policies & guidelines documentation for GBP owners. The following sections were added, almost doubling the amount of documentation:

• Dedicated landing pages
• Direct action completion
• Business links crawlability policy (this is the longest new section by far)

Here are important bits:

• For businesses with multiple locations, action links must lead to a website for a specific location.

◦ (Don't link to a general landing page, or the wrong location.)

• Local business links must allow customers to complete the designated action.

◦ (If the link text says "order", they'd better be able to place an order)

• Links that cannot be crawled by our automated tools may be removed.

◦ (Don't block googlebot, don't require a CAPTCHA, etc - more in documentation)

It all seems pretty reasonable, but if you notice any links disappearing from GBPs this would be a good place to start your investigation.