Thunderbird automatically marks new emails as read

Hello, I am having an issue with Thunderbird automatically marking new emails as read. When a new email arrives, it initially appears as unread in Thunderbird, but after a short time it automatically changes to read, even though I have not opened or clicked on the email. I use Thunderbird on two computers – a desktop computer at the office and a laptop when working from home. The problem started after I installed and configured Thunderbird with the same email account on my laptop. I have already disabled the “Automatically mark messages as read” option in Thunderbird. I have also repaired/reindexed the Inbox folder, but the problem remains. The email account is configured using IMAP on the laptop. An important detail: when I check the same emails in Gmail through a web browser, they are still marked as Unread. Only Thunderbird shows them as Read. Could using the same email account in Thunderbird on two different computers be causing this issue? Could there be a synchronization or local Thunderbird setting that changes the read/unread status? Could you please advise what I should check or change to fix this? Thank you.