I'm having the same issue with Message+ on two separate devices, both updated to current stock software deployments. The issue started about 4 weeks ago. Message+ sits on the notification bar until the phone is rebooted. Eventually it returns after a few hours. The notification is this:

Message+

Syncing messages

Temporary background processing

App data:

Version 6.6.7 (latest available on Play Store)

Phone data:

Note 8

Model SM-N950U1

Baseband N950U1UES5CRH7