Custom Function works, but not when invoked with column names

Try using this version of your function instead. It avoids reusing the inputs as variables inside the function.

(inputTotalLoops as number, inputLoopNumber as number, inputCurrentValue as text, inputEndValue as text) =>
let
    varTotalLoops = inputTotalLoops,
    varCurrentLoop = inputLoopNumber +1,
    varEndValue = inputEndValue & Text.From(Number.From(inputCurrentValue)-1) & "/" & inputCurrentValue & ",",
    varCurrentValue = Text.From(Number.From(inputCurrentValue)+1),
    output =
    if varCurrentLoop >= inputTotalLoops
    then Text.Start(varEndValue, Text.Length(varEndValue)-1)
    else @testFunction1(varTotalLoops, varCurrentLoop, varCurrentValue, varEndValue)
in
    output

Pat