Just double-checked and I do not have the numbering problem in my merges. I suspect it is because I use outline numbering tied to styles as set out in How to create numbered headings or outline numbering in Ribbon Versions of Word by Shauna Kelly rather than a simple numbered list.

Just tried a simple list as well and no problem, it restarted for each new record.
I just tried telling the list to restart and still no problem. What kind of numbering are you using? A SEQ field perhaps?