Dynamic Spill-down Formula Fails to Spill

In an Excel 365 environment, I attempt to insert a dynamic spill-down formula:

Sub WhyDoesThisFail()
    Range("A1").Formula = "=SEQUENCE(10)"
End Sub

Strangely I get:

There is a gratuitous @ and no spill. (If I enter the formula manually, I get the expected result.)

Equally strangely, if I enter the formula as a String I get the expected result!

Sub WhyDoesThisWork()
    Range("A1") = "=SEQUENCE(10)"
End Sub

Why does .Formula fail?? Can anyone explain what’s going on??