Runs once, then never again - excel

Question

I posted a question to Stack Overflow about creating a Excel Macro to add/subtract a value in a column from the previous column, then clear the current column.
Excel cell value update macro
The code worked find for years in office 2007, but no longer works in office 365 (2016/2019).
If F1 has a value of £100.00, and I type -50 into G1, F1 changes to £50.00 and G1 is empty.
Each cell in column F is changed by an amount entered into the row in column G.
In Office 365 versions of Excel this macro executes once then won't run again until I close an re-open the document. Has something changed about the way macros work that would cause this?
Edit:
Here is the code (attached to sheet 1), slightly modified from my previous question to work on column G.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim FirstNum As Currency 'Long is for number currency should help keep format
Dim SecNum As Currency
If Target.Column = 7 Then 'Only Runs if Cel l being changed is in column C, Might need to be Columns
FirstNum = Target.Offset(0, -1).Value ' Value in Column B
SecNum = Target.Value ' Value being typed in C
Target.Offset(0, -1).Value = FirstNum + SecNum ' Makes Cell B equal to difference of previous value and value typed in C
'MsgBox ("Difference Found") ' Just to display code worked Remove when confirm code works
Target.Clear ' Clears Value you typed
End If
End Sub

Answer 1

I don't have enough rep to do a comment...
Without seeing your full code it's hard to know. But if your code has
application.enableEvents = false
You will need to to set it back to True eventually. Or worksheet change events won't fire off until the workbook is closed and opened again

Answer 2

I think it is nothing wrong with Office 365...
In order to work as you explain, the code must look like:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim FirstNum As Currency
Dim SecNum As Currency
If Target.Column = 3 Then 'Only Runs if Cell being changed is in column C, Might need to be Columns
FirstNum = Target.Offset(0, -1).Value ' Value in Column B
SecNum = Target.Value ' Value being typed in C
Target.Offset(0, -1).Value = FirstNum - SecNum ' Makes Cell B equal to difference of previous value and value typed in C
Target.Clear
End If
End Sub
But, you must understand that this code works only for changing cells in column C:C.
The variant you received six years before worked only in the same column, but on vertical. I mean, changing of C2, it used to modify C1. But it had a bug. If you try modifying of C1 it will try to set an Offset of -1, row zero does not exist in Excel and returns an error...
I cannot understand how the event functioned at least once for ranges G1, F1. It could work only if some links exists in the sheet, or you adapted the code and in such a case the real used code will give us the possibility to analize and tell you, not supposing, where the problem is. I do not think it is a matter of Office version, but who knows...

Answer 3

It sounds like a formula would suffice. However a cell cannot refer to itself, you could try,
E1: =F1+G1
Where E1 is a new cell with the sum of F1 and G1, because you are entering its sign operator i.e. - or + the you can leave it as addition in cell E1. To use this for a column just drag from the lower left square on the cell E1 all the way down to the row number you want. This will refresh every time a value changes.
excel image