I made a quick & dirty modification to fix assignees for MRs:

require 'json'
require 'net/http'
require 'uri'

source_instance = {
    uri: URI('https://gitlab.com'),
    access_token: '<insert token here>'
  }
  
  target_instance = {
    uri: URI('http://localhost:3000'),
    access_token: '<insert token here>'
  }
  
  namespaces = [
    '<some namespace>',
    '<some namespace>/<subgroup>',
  ].map{ |namespace| namespace.sub('/', '%2F') }  

def api_request(instance, method, endpoint)
  unless instance[:http]
    instance[:http] = Net::HTTP.new(instance[:uri].host, instance[:uri].port)
    instance[:http].use_ssl = true if instance[:uri].scheme == 'https'
  end

  request = method.new(endpoint)
  request['PRIVATE-TOKEN'] = instance[:access_token]
  instance[:http].request(request)
end

$target_user_ids = {}
def lookup_user(instance, username)
  unless $target_user_ids[username]
    user = JSON.parse(api_request(instance, Net::HTTP::Get, "/api/v4/users?username=#{username}").body).first
    if user
      $target_user_ids[username] = user['id']
    else
      puts "Could not find user @#{username} on #{instance[:uri]}!"
    end
  end

  $target_user_ids[username]
end

namespaces.each do |namespace|
  projects_endpoint = "/api/v4/groups/#{namespace}/projects"
  project_pages = api_request(source_instance, Net::HTTP::Head, projects_endpoint)['X-Total-Pages'].to_i
  project_pages.times do |project_page_index|
    projects = JSON.parse(api_request(source_instance, Net::HTTP::Get, "#{projects_endpoint}?page=#{project_page_index}").body)
    projects.each do |project|

      puts "Fixing assignees for Issues in #{project['name_with_namespace']} ..."

      issue_pages = api_request(source_instance, Net::HTTP::Head, project['_links']['issues'])['X-Total-Pages'].to_i
      issue_pages.times do |issue_page_index|
        issues = JSON.parse(api_request(source_instance, Net::HTTP::Get, "#{project['_links']['issues']}?page=#{issue_page_index}").body)
        issues.each do |issue|
          puts "Fixing ##{issue['iid']} (#{issue['title']})"
          assignee_ids = issue['assignees'].map {|user| lookup_user(target_instance, user['username'])}.reject {|id| id.nil? }

          if assignee_ids.empty?
            puts 'Issue has no assignees!'
            next
          end

          puts "Setting assignees to #{assignee_ids}"
          target_endpoint = "/api/v4/projects/#{namespace}%2F#{project['path']}/issues/#{issue['iid']}?assignee_ids=#{assignee_ids.join(',')}"
          response = api_request(target_instance, Net::HTTP::Put, target_endpoint)
          puts "Failed! #{response.body}" unless response.kind_of? Net::HTTPSuccess
        end
      end

      puts "Fixing assignees for Merge Requests in #{project['name_with_namespace']} ..."
      
            mr_pages = api_request(source_instance, Net::HTTP::Head, project['_links']['merge_requests'])['X-Total-Pages'].to_i
            mr_pages.times do |mr_page_index|
              merge_requests = JSON.parse(api_request(source_instance, Net::HTTP::Get, "#{project['_links']['merge_requests']}?page=#{mr_page_index}").body)
              merge_requests.each do |merge_request|
                puts "Fixing ##{merge_request['iid']} (#{merge_request['title']})"
                assignee_id =  lookup_user(target_instance, merge_request['assignee']['username'])
                
                if assignee_id.nil?
                  puts 'Merge Request has no assignee!'
                  next
                end
      
                puts "Setting assignee to #{assignee_id}"
                target_endpoint = "/api/v4/projects/#{namespace}%2F#{project['path']}/merge_requests/#{merge_request['iid']}?assignee_id=#{assignee_id}"
                response = api_request(target_instance, Net::HTTP::Put, target_endpoint)
                puts "Failed! #{response.body}" unless response.kind_of? Net::HTTPSuccess
              end
            end

    end
  end
end

@abuango merge requests should not be affected by this—at least I could not reproduce there. 🤔

Uhmmmm, I tried it and the Administrator account on my instance was assigned, despite having the same username on the instance. Also, this only works on a public project (on the Target instance).

@abuango Oh... sorry... maybe the IMPORT is broken. I only tested the export. 😟