Excel and LibreOffice behave differently.

merge_cell_and_border.rb

# -*- coding: utf-8 -*-
# frozen_string_literal: true
require 'axlsx'
package = Axlsx::Package.new
package.use_shared_strings = true # for Numbers
wb = package.workbook

gridstyle_border = wb.styles.add_style border: { style: :thin, color: 'FFCDCDCD' }
wb.add_worksheet do |sheet|
  sheet.sheet_view.show_grid_lines = false
  sheet.add_row []
  sheet.add_row [nil, 'foo', 'bar', 'baz'], style: [nil, gridstyle_border, gridstyle_border, gridstyle_border]
  sheet.merge_cells 'B2:D2'
  sheet.add_row []
  sheet.add_row [nil, 'foo', 'bar', 'baz'], style: [nil, gridstyle_border, nil, gridstyle_border]
  sheet.merge_cells 'B4:D4'
end

package.serialize(File.basename(__FILE__).sub(/\.rb\z/, '.xlsx'))

merge_cell_and_border.xlsx