It appears to be bug in latest version. The Sent mail will not sync at all.
Only way to sync to latest is to Repair the folder.
Removing the account and resetting has no effect. It has to be repaired after every Sent mail. Log shows no errors, only shows its syncing.
All other clients O365, Apple work well - no issues.
It started with last update to 10.0.3530 (f0b89b7).