This occurs when the “New Jira Ticket View” is enabled in a user’s profile (regardless of New or Old Jira Experience choice). A time estimate is entered, accepted, then deleted quickly after with no further interaction from the user.

The issue is described in more detail in this community post: https://community.atlassian.com/t5/Jira-questions/The-quot-Time-Estimate-quot-field-resets-to-quot-None-quot-after/qaq-p/764577

We have a current workaround which is to turn off the New Jira ticket view, but would like to know if this view is part of the mandatory rollout of the new Jira Experience in two week's time.

Disabling "Jira legacy mode" in time-tracking settings allows time estimates to be set using New Jira ticket view. However, issues can not be set with time estimates using days as a value, for a currently unknown reason.  A warning icon is shown in the time estimate entry field and a cropped error message which cannot be read appears.