I can only see yesterdays emails in my sent folder, else they automatically move to 2026 a sub folder in Archive 2

I have been using Thunderbird since over 7years, facing issue since few days. Have my personal mail linked to my website hosted on a unix server. All sent mails were earlier visible in Sent folder. But now since a week only yesterday's mails are visible. All previous mails are in a subfolder by year 2026 in Archive. This makes it difficult to search sent emails. While this is not an issue for the Inbox where I can see emails from 2017. Please suggest how can I restore all my previously sent emails since always, to my Sent folder.