This page identifies a possible issue, concern, error, limitation, or bug!
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To say the least, the Cell > Grouping > Automatic Grouping mode of Mathematica is "hit and miss". Try Cell > Grouping > Manual Grouping instead, and make it part of your workflow. You are going to have to review and double-check the groupings made (imposed) by Automatic Group anyway, might as well save yourself time and get it right manually.
Activating Automatic Cell Grouping mode can have side effects and there is no UNDO for the Notebook for such settings!