Hi Marie-2019,
Based on my test, in SharePoint online list, when using fill handle in edit mode, it does not increase the number. It simply
copy-paste the above value. This is by design.
As you mentioned, that a user reported that it used to work when dragged down and now the behavior has been changed, in this case, I’ll suggest you provide your feedback in SharePoint UserVoice. UserVoice is the best way to give the feedback to Product Group as it’s actively monitored by them and many new features have been implemented from our customers feedback.
We highly appreciate your understanding.
Regards,
Neha