This may not be an entirely Atlassian/Jira thing but here's the behavior that's been witnessed.
One someone would key values into various text fields (summary and customer fields) the entry would be cached so that the next time this is entered, it's doing a text look up. This is a browser feature. But recently people have been reporting that this is no longer available on the site.
Is this something that was disabled by Atlassian or is there some other explanation as to why the feature has stopped?