References to external JIRA issues causes ActiveRecord::AssociationTypeMismatch

https://sentry.gitlap.com/gitlab/gitlabcom/issues/12216/

ActiveRecord::AssociationTypeMismatch: Issue(#70144146175180) expected, got ExternalIssue(#70144182266560)
  app/models/merge_request.rb:535:in `block (2 levels) in cache_merge_request_closes_issues!'
    self.merge_requests_closing_issues.create!(issue: issue)
  app/models/merge_request.rb:534:in `each'
    closes_issues(current_user).each do |issue|
  app/models/merge_request.rb:534:in `block in cache_merge_request_closes_issues!'
    closes_issues(current_user).each do |issue|
  app/models/merge_request.rb:532:in `cache_merge_request_closes_issues!'
    transaction do
  lib/gitlab/metrics/instrumentation.rb:155:in `block in cache_merge_request_closes_issues!'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
...
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