If you add the Time Tracking field to any of the workflow screens, you are able to enter a empty value for the remaining estimate. When you set the remaining estimate via the Log Work screen, it disallows an empty value.

Steps to reproduce:

1. Enable Time Tracking
2. Add Time Tracking field to Workflow Screen / Resolve issue screen
3. Create an issue and set the estimated time
4. log work on the issue
5. Perform a workflow transition on that issue that will show a screen with the time tracking field, leave remaining estimate empty
6. JIRA will accept the input and perform the transition, remaining estimate will show as "Not Specified".

JIRA should actually reject the empty remaining estimate as per the Log Work screen.

The workaround is to not add the time tracking fields to screens and use the Log Work feature instead.