Investigate why alarms are triggered when adding team members with different domains to a form

## Context

### [#incident-2025-11-14-connection-closed-and-request-timed-out-errors](https://app.slack.com/client/T2G2S06PM/C09T5EV102G)

**Incident report** https://docs.google.com/document/d/1_m741vCu3nieVoJFrkVyGqmKTx4aEEvLoA2L_p0MXaI/edit?tab=t.0

**Question** Is there something about adding people from other organizations (not a self-serve action, not validated) that might actually be the trigger here?

**From Support** Everything seems to work "fine" from the UI, but then there are many errors in [#forms-production](https://app.slack.com/client/T2G2S06PM/C05G766KW49)

Start with the linked incident report and the #forms-production and #incident-2025-11-14-connection-closed-and-request-timed-out-errors discussions, then reproduce adding team members from different domains to a form. Trace the resulting alarms and errors to determine whether this action is the trigger, and document the confirmed cause or ruled-out hypothesis.

Written by the indexing model from the issue text.