Personal Macro Workbook interference with other workbook macro(s)

I've just started using a Personal Macro Workbook(PMW). I have to because I can't put macros in shared network workbooks (of course!).

I have 3 extremely simple macros in my PMW. I'm talking REALLY simple. I don't see how they could interfere with anything else. Here's an example of one:

Sub CommentLate()
'
' CommentLate Macro
' Inserts Comment in active cell "Late, called at <time>."
'
' Keyboard Shortcut: Ctrl+Shift+C
'
ActiveCell.AddComment
ActiveCell.Comment.Visible = False
ActiveCell.Comment.Text Text:="Jay:" & Chr(10) & "Late, called at " & Format(Now(), "h:mm am/pm") & "."
ActiveCell.Select

End Sub

It works perfectly (yeah, I know, no declaration of variables). The other two are equally simple.

However, after I created the PMW, I discovered that I could no longer run a specific macro (M1) from an established shared workbook file that I must use. It previously worked before I introduced the PMW. There is no error message; it just doesn't produce an output. If I close the PMW, it works again. Note that at least one other macro in the shared workbook works just fine, so it's not universal interference/corruption.

The really, really weird thing is this: While the PMW and this file are open, I tried the following experiment (trying to duplicate partially the function of M1). This experiment was in a new workbook entirely.

In cells A1 to A4 I put the numbers 1 to 4. In cells B1 to B4 I put ";". In C1 I put "A1&B1". It produced the expected result: "1;". However, when I copied C1 to C2-C4, the results were all "1;" for all four cells. When I checked the formulas that were copied, they appeared as expected: "A2&B2", "A3&B3", "A4&B4". But the results were still showing all "1;".

This behaviour disappears when I close the PMW and I get the expected results: 1; 2; 3; 4;. And it reappears when I reopen the PMW.

Any ideas? Not really expecting any solutions, but hey, I thought I would ask...

Xan