As an update, I see where the problems start, which is in Col P of a
given sheet (A thru O are fine), about 615 rows down. This is
meaningful b/c I used some test data to develop the xlsx file that was
this many rows down. To prepare for the largest data sets I'll be
inputting once the workbook functions properly, I later made it so all
formulas are present to row 6000 and only return a value if the data I
input goes that far down. But there's no reason for it not to update
that I know of.

The formula for each cell (from row 2 to 6000) in Col P is {=MAX(IF(K
$1:K$6000=K467,O$1:O$6000,""))}, where this example is the formula for
cell P467. K467 in the formula would be K468 at cell P468.

This formula is obviously an array formula but is a different array
formula for each single cell in Col P (that is, I only have one cell
selected when I press Ctrl-Shift-Enter). I can only assume it's just
too large for Excel to handle 6000 different arrays in one column,
even though the formula itself is pretty simple. Anyway, this was my
attempt at getting something like a MAXIF function. If anyone has any
better ideas I'd appreciate it!

Thanks,
K