Removing Rows with Certain Values

The issue in your code is the use of the or operator in your Text.Contains conditions.

each not Text.Contains([Cust PO Number], "JAN") or not Text.Contains([Cust PO Number], "JANUARY")

This will always return true unless the value contains both "JAN" and "JANUARY" at the same time:

If [Cust PO Number] contains "JAN" but not "JANUARY", the second condition is true, so the whole expression is true.
If it contains "JANUARY" but not "JAN", the first condition is true, so again the whole expression is true.
So the row is not removed.

You should use and instead of or to ensure the row is removed if it contains either "JAN" or "JANUARY".

#"Remove CONS - JANUARY" = Table.SelectRows(#"Remove 0 Price", each not Text.Contains([Cust PO Number], "JAN") and not Text.Contains([Cust PO Number], "JANUARY")),
#"Remove CONS - FEBRUARY" = Table.SelectRows(#"Remove CONS - JANUARY", each not Text.Contains([Cust PO Number], "FEB") and not Text.Contains([Cust PO Number], "FEBRUARY")),

Please give a thumbs up if this fixes your issue.