Same small group of external members repeatedly, silently removed from Teams group chat — not tied to Guest vs. External status

I manage a Teams group chat (business tenant, 64 members across ~37 external domains) that's experienced repeated, unexplained removals of the same handful of external members over nearly two months — July 1, 13, 14, 16, 20, 22, 23, and 24.

Key details that rule out the usual explanations:

• It's always the same small group — 5 specific individuals across 4 different external domains (not a rotating or random set from the 60+ members). Two of the affected individuals share the same domain, which makes me suspect this may be domain-level rather than purely account-level — I'd expect any additional user from those same domains to be affected too, though I haven't tested that directly.
• Doesn't appear tied to Guest vs. External status. I'm partway through bulk-inviting members as B2B guests via Entra ID (not yet complete), and removals have happened both before and during that process — so this doesn't look like something that'll resolve just by converting people to Guest status, though I can't fully rule that out until the conversion is complete.
• The chat's own activity log shows the removal as unattributed — every deliberate add/remove I perform is logged as "Claudia Catlett removed/added X," but these recurring removals show only "[Name] and 4 others were removed from the chat," with no actor named at all.
• Purview Audit search for ChatMemberRemoved returns zero results for these events, despite MemberAdded logging correctly for the same chat.
• I'm the chat creator, hosted under my own tenant (confirmed via activity log), so this isn't the "creator account disabled" scenario some other threads describe.

I have an active Microsoft support ticket open on this, but it's been slow-moving. Posting here in case anyone's seen this specific pattern — a consistent small group getting silently ejected regardless of Guest/External status, chat-wide federation otherwise stable — rather than the more commonly reported "guest access breaks broadly" issue.

Happy to share screenshots of the activity log showing the unattributed removal pattern if useful.