Custom tags disappearing in all folders after switching views (since Sept 28, 2026)

Dear Thunderbird Support Team, I am writing to report an issue with custom message tags in Thunderbird that started occurring around September 28, 2026 (likely following a recent update). Issue Description: The problem applies to all folders (Inbox, Sent, custom subfolders, etc.) and is strictly isolated to user-created (custom) tags. When I apply a custom tag to any email, the tag visually attaches to the message. However, as soon as I switch to another folder in the folder pane and then return to the original folder, the custom tag disappears completely. Key Observations: No Relation to Filters: This issue happens with regular emails in any folder, with no filters involved. Custom Tags vs. Default Tags: This affects only custom tags created by the user. Default built-in tags (e.g., Important, Work, To Do) save correctly and persist after switching folders. Timeline: All custom tags assigned before September 28, 2026, are still preserved correctly on older emails. The issue affects any new attempt to set a custom tag after this date. Steps to Reproduce: Select any email in any folder. Apply a user-created (custom) tag to the email. Observe that the tag is visually assigned. Click on any other folder to switch views. Return to the original folder — the applied custom tag is gone. Could you please assist in resolving this issue or suggest a fix/workaround? Custom tags are essential for our daily email management. Thank you for your help and support. Best regards, Ihor Hirnyi E-mail: ig@zrn.com.ua