I am also having this issue. Textra is the only app I have where this happens. In order to clear the notification I have to go into the Smartwatch app, click on the notification button and remove Textra. Selecting the notification window on the watch and clicking "Dismiss all" does not clear the notification. Marking all as read on the app from the phone does not work either. Even turning off and restarting the watch does not clear the Textra notification. This is really a pain and has caused me to go back to the stock messaging app for the time being, even though I've purchased the Pro version of Textra. Please look into it.

For troubleshooting purposes: it may have something to do with how Textra handles notifications, because when I am on the watch viewing a Textra notification the options I have are "Reply", "Open in Phone", "Block", or "Mark as Read". However, the stock messaging app has "Reply", "Open in Phone", "Block", and "Dismiss" as it's options. All the other app notifications (Mail, WhatsApp, Facebook, Twitter, etc.) also have "Dismiss" and not "Mark as Read". Maybe it is the difference between "Mark as Read" and "Dismiss" is causing the problem?

Phone: Sony Xperia Z3v
Android Version: 4.4.4
Textra Version: 3.0 (Pro user)
SmartBand Talk version: 6.0.0.108