Is it better to pay extra to my mortgage each month, or borrow against my heloc (apply the principle to the mortgage), and pay $1200 to the Heloc along with normal payment to the Mortgage note?

So here is my question in detail. I have created a spreadsheet that analyses
two loans versus one. The first is my straight 30 year note at 3.875. By
paying the $1200 extra each month it will be paid off in February 2029. The
second is my HELOC (5.875%), where if I borrow 87000 (applied as a one time
payment to the mortgage), the $1200 paid to the HELOC, along with paying my
minimum mortgage payment (2 loans essentially), shows them both paid off in
August of 2027 (a saving in combined interest of over $2000, and paid off a year
and a half early.
Intuitively this does not make sense, but I do not see the error in my
spreadsheet.
Has anyone ever looked into this? I am happy to forward the excel spreadsheet
to anyone who wants to see this unexpected result.