VLOOKUP returning incorrect results

I have a large complex workbook that I've been using for years with great success. But all of a sudden I'm getting incorrect values.
Here's the situation.

One tab in the workbook (let's call it ACCOUNT) has dates in column A and amounts in column C.

There's a range ACCT_PMT that's defined over the 1st 3 columns and all rows in ACCOUNT.

A different tab (call it PAYMENT) also has dates in column A. In its column C, I have the formula
=VLOOKUP(Annn,ACCT_PMT,3) - where nnn is the current row number. 

For instance, in cell C35 it would be =VLOOKUP(A35,ACCT_PMT,3)

Suddenly within the last few days the cells in PAYMENT column C are NOT returning the appropriate values, and it's not obvious where they're getting the values they display.

Example: ACCOUNT cell A18 has 09/15/22, and cell C18 has 517.26

PAYMENT cell A35 has 09/15/22, so I would expect the formula in C35 to return 517.26

But it doesn't. and I nowhere in the ACCOUNT tab do I see the value that's displayed in PAYMENT's C35.

As I say, I've been using this workbook for years, and have never encountered anything like this until the last few days.

Any idea what's going on?