Table.ReplaceValue list not replacing null data consistently

In this example, the nulls are not empty strings or actual blanks, they are text "null". However, replacing "null" with null did recreate your issue. Here's a simplified version of what you provided:

let
  Source = Table.FromRows(
    Json.Document(
      Binary.Decompress(
        Binary.FromText(
          "i45WckwpSy0qySzOzEtX0lEqTi5IAVJ5pTk5SrE60UoBiUUlealF+s75ecWlOSWJeSUK7qlAgcSSVJC6MBMF54zU5GyFoNTkzILM1LwSZM14jI4FAA==",
          BinaryEncoding.Base64
        ),
        Compression.Deflate
      )
    ),
    let
      _t = ((type nullable text) meta [Serialized.Text = true])
    in
      type table [LeadSource = _t, Lead_Source_Detail__c = _t, #"Person Source" = _t]
  ),
  #"Replaced Value" = Table.ReplaceValue(
    Source,
    "null",
    null,
    Replacer.ReplaceValue,
    Table.ColumnNames(Source)
  ),
  #"Person Source Replace null" = Table.ReplaceValue(
    #"Replaced Value",
    null,
    each
      if [Person Source] = null and List.Contains({"Advertising"}, [LeadSource]) then "Digital Advertising"
      else if [Person Source] = null and Text.Contains([Lead_Source_Detail__c] = "conference") then "Event"
      else if [Person Source] = null then "none"
      else [Person Source],
    Replacer.ReplaceValue,
    {"Person Source"}
  )
in
  #"Person Source Replace null"

The problem is with

Text.Contains([Lead_Source_Detail__c] = "conference")

On its own, this throws the error "Expression.Error: 1 arguments were passed to a function which expects between 2 and 3".

I haven't dug into why just eats the error when included inside else if instead of returning an error for that row, but all that's needed to resolve it is to replace "=" with ",".

Text.Contains([Lead_Source_Detail__c], "conference")