Not receiving e-mails from a specific vendor again.

Periodically, Comcast decides to secretly reject e-mail from a vendor as spam _before_ it hits my mailbox and my spam filters.  This results in my e-mail being secretly put on a no-send blacklist by the vendor's mail software.  The vendor sees mail go out but I see nothing come in.  Because this is all done in secret with no notification, it's a fight to recognize and resolve on my own because, of course, support is no help.

Now I'm having a similar problem with a different vendor.  This vendor has a ".store" domain but, looking at the headers, is actually coming through Google/GMail.  I got a payment request e-mail for a preorder on November 16th.  I paid on November 25th and it shipped on November 16th but I did not receive either e-mail notification.  They sent a payment request for a different preorder December 1st but I did not receive that either.

I just sent myself an e-mail from GMail and it got through.

The only difference between the last e-mail that I received from them (Nov 16th) and my test e-mail (Dec 2nd) is:

Their e-mail header: 

X-Xfinity-Message-Heuristics: IPv6:N;TLS=1;SPF=1;DMARC=
Authentication-Results: resimta-a2p-650767.sys.comcast.net;
 dkim=pass header.d=showz.store header.b=Xh/RNxIS

My e-mail header:

X-Xfinity-Message-Heuristics: IPv6:N;TLS=1;SPF=1;DMARC=P

Authentication-Results: resimta-c2p-560315.sys.comcast.net;
 dkim=pass header.d=gmail.com header.b=COnTjLPr

I saw the pinned message about DMARC being changed in July 2024 but that should've been in effect long before now.  They're not spoofing a domain either - their ".store" domain is all I see except the initial hop though the Google/GMail SMTP relay.  What changed that started blocking these e-mails a week before Thanksgiving 2025?  Is there another secret layer of filtering getting in the way?  What do I tell the vendor that they might need to fix?

::sigh:::  remember when e-mail just worked?