Max loss by month

The issue is the numbers under column J. Since the Match is looking up the last value in J and that's on a lower row than the last value in B, you get differing data ranges. You have three options:
1. Move the data under J somewhere else.
2. Adjust the ranges manually.
3. Change the last Match to read off B:B instead of J:J. Note that any data put under B might then throw off your calculations or result in an error, depending on what was in that data and the corresponding value in J.

I don't see any change to your conditional formatting. It's working for me.

I didn't bother checking the first time whether your data was from formulas or not. For J, since it is a formula, adding 0 or multiplying by 1 makes no difference, since any formula will try to coerce a number stored as text into a number. For the dates, it might have made a difference, but if this is your real spreadsheet, it appears Excel is reading those correctly as numbers.

I suspect that formula will be ok... here the entire column ref. is used to create a range and that element should function without issue.
(note the *may* in the warning - if in doubt test)

However, what I would say is the formula is a little odd - unless all the Months match the criteria the result would always be 0 (assuming J > 0 - not checked)

You might find you actually want to use:

=MIN(IF(MONTH(Main!B5:INDEX(Main!B:B,MATCH(99^99,Main!B:B)))=MONTH(1&C1),Main!J5:INDEX(Main!J:J,MATCH(99^99,Main!B:B)))
confirmed with CTRL + SHIFT + ENTER

note I also reverted the final match range to B so as to ensure both ranges share the same dimensions irrespective of content in J [edit: this is a point made by darkyam in an earlier post]

@Lewis Koh - repeating a point made in an earlier post:

Originally Posted by D.O

I suspect that formula will be ok... here the entire column ref. is used to create a range and that element should function without issue.
(note the *may* in the warning - if in doubt test)

I draw your attention to the word "may".

Does Feb!E5 work ? Yes.