This sounds like a spam filter that is catching some of the emails sent through Keap. The fact that they are receiving most emails tells me that they can get them just fine and the issue is unlikely to be on our/Keap's end.
The parts that make this a tad bit hard to troubleshoot/fix as a sender-
1. Keap sends the email not LU so we have no control over that process.
2. Once an email is sent we have no control to make the recipient receive it. Think of it like a paper airplane, once you throw it, you no longer control it.
So, with all that said, this issue could be due to a change in your Junk mail settings, among other things. Below is a list of the settings that you can check.
1. Check your Junk folder for missing emails.
2. Make sure your mailbox isn't full or close to its limit.
3. Check your Inbox filter and Sort settings.
4. If Focused Inbox is enabled, check your Other tab for the missing emails.
5. Check your Blocked senders and Safe senders list (linked below).
6. Check your email rules.
7. Check to see if email Forwarding is enabled.
8. Check if your account is blocked.
1. Check your Junk folder for missing emails.
2. Make sure your mailbox isn't full or close to its limit.
3. Check your Inbox filter and Sort settings.
4. If Focused Inbox is enabled, check your Other tab for the missing emails.
5. Check your Blocked senders and Safe senders list (linked below).
6. Check your email rules.
7. Check to see if email Forwarding is enabled.
8. Check if your account is blocked.
They can also go to support.microsoft.com to try and look up other resolutions. This is an example of one that might help in this situation, add-recipients-of-my-email-messages-to-the-safe-senders-list-