1) The cause of your issue is most likely Cleanup, Battery, Speed Improvement type apps, for example Cleanmaster, CM Suite, CCleaner, Battery Doctor, Greenify, Amplify and apps of this type.
You need to add Textra to the excluded list inside the culprit apps settings (if available).
2) Also some phones stock battery saving preferences can cause an issue, check as follows:
Huawei: Android Settings > Battery > 'Close apps after screen lock', turn OFF the entry for Textra.
Samsung S7 phones: Android Settings > Battery > MORE (top right) > says 'Optimize battery usage', locate Textra and turn OFF.
HTC 10: Android Settings > Power > Battery optimization > Textra > Don't optimize
Xiaomi: Android Settings > Battery & Performance > Manage apps battery usage > Choose apps > Select Textra > In background settings set to No restrictions.
Other Samsung Phones: Android Settings > Battery > App power saving [DETAILS], locate Textra = Off / Disabled
Other Phones: Android Settings > Battery > Settings / Cog or dots icon (top right) > Battery Optimization > All apps > Textra = Don't Optimize
3) Asus phones: Android Settings > Auto Start Manager [DOWNLOADED], set Textra to Allow.
4) Elephone or Blackview: Android Settings > Background task clear = Off (or add Textra to the whitelist)
Important: Please check carefully and report back, this will help us sort out your issue and help us to help others too.