Excel Script - not calculating as expected

You get Show/Hide values at very beginning, later you change MonthNumber, Show/Hide within grid are recalculated, but in array for show/hide are still old values. You need to get it from the grid again, or get just befor you will use it.

Not sure I understood entire logic, thus tried to reproduce as

function main(workbook: ExcelScript.Workbook) {

    const sheetRefs         = workbook.getWorksheet("Refs")
    const rangeCurrentMonth = sheetRefs.getRange("CurrentMonth")
    const sheetYear          = workbook.getWorksheet("Year")
    const rangeShowHide      = sheetYear.getRange("ShowHide")

    let MonthNumber: number = +rangeCurrentMonth.getValue()

    MonthNumber > 1 ? MonthNumber -= 1 : ''
    rangeCurrentMonth.setValue(MonthNumber)
    sleep(0.1) // in general not necessary

    const rangeShowHideValues = rangeShowHide.getValues()
    const colCount: number = rangeShowHideValues[0].length

    for( let col of Array.from(Array(colCount).keys() ) ) {
        rangeShowHide
            .getColumn(col)
            .setColumnHidden( rangeShowHideValues[0][col] === "Show" ? false : true ) 
    }
}
// Delay in seconds
function sleep(seconds: number) {
    const waitUntil = new Date().getTime() + seconds * 1000
    while (new Date().getTime() < waitUntil) { }
}

for the attached file. Not sure if scripts waits for the greed recalculation or not, added some delay. However, it works exactly the same without it.