core/region: fix sub tree may not consist with root tree

[nolouch]

What problem does this PR solve?

Issue Number: Ref #7897

What is changed and how does it work?

core/region: fix the sub tree may not consist of root tree

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• rleungx

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.36%. Comparing base (36a1fa2) to head (2236ffb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8187      +/-   ##
==========================================
- Coverage   77.39%   77.36%   -0.03%     
==========================================
  Files         471      471              
  Lines       61354    61353       -1     
==========================================
- Hits        47484    47468      -16     
- Misses      10311    10329      +18     
+ Partials     3559     3556       -3     

Flag	Coverage Δ
unittests	77.36% <100.00%> (-0.03%)	⬇️

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[JmPotato]

/merge

[ti-chi-bot]

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 2236ffb