iCloud Mail custom domain: Intermittent spam filtering for a specific VIP sender

I am experiencing a persistent and frustrating issue with my custom domain email address set up on iCloud Mail. For some reason, emails from a single client are randomly being sent to my Junk folder, while other emails from the same client are delivered to my inbox without any problem. All other senders work fine.

I have already taken the following steps:

• The sender is in my Contacts.
• I have marked the sender as a VIP.
• I have manually marked the junked messages as "Not Junk" on iCloud.com multiple times.

Despite these measures, the problem continues. This seems to happen with certain messages - that look seemingly no different from others, but not with every email from this person.

My main question is: If I have a sender in my contacts and have designated them as a VIP, shouldn't their messages be exempt from spam filtering? The fact that the iCloud server-side filter is still flagging these messages seems counterintuitive to the purpose of the VIP feature.

Has anyone else experienced this kind of intermittent junk filtering for a VIP sender on a custom domain? I'm trying to figure out if this is a known bug or if there is something specific about the content of these messages that is triggering the filter.

Any insights or suggestions would be greatly appreciated. Thank you!