Noticed if you edit an existing task note, the subject is blank and can’t save without new subject?
Yes, this is a known issue
https://github.com/salesagility/SuiteCRM/issues/5327
there is a fix there if you need it immediately, or you can wait for the next releases early next week.
mysql Ver 14.14 Distrib 5.7.21, for Linux (x86_64) using EditLine wrapper
PHP 7.0.22-0ubuntu0.16.04.1
Apache/2.4.18 (Ubuntu)
Ubuntu 16.04 LTS