If the issue is updated, it is marked as unread and opening an issue will mark it as read.
Manually change Issue read indication
Open an Issue and click on More to 'Mark issue as unread'.
Custom JQL Function
Issue Read Status provides a custom JQL function to easier find unread issues.
Disable Issue Read Status
Click on your profile picture and select Profile. Locate 'Read Issue Status' to disable the app for your profile.