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. 

issue not in readIssues()

Disable Issue Read Status

Click on your profile picture and select Profile. Locate 'Read Issue Status' to disable the app for your profile.