Spell check; do not check spelling box turning on automatically

A filled box next to the "Do not check spelling or grammar" option could mean conflicting settings in the current selection, that is, the option is turned on for some parts of the selection and turned off for other parts.

Usually, though, the filled box means that the selection is too large for Word to correctly display the status.

If you select just a single word or sentence for which Word fails to check the spelling, is the "Do not check spelling or grammar" option checked or cleared?

When error occurs I firstly turn off check box without selecting text;

doesn't catch the typos.

Then I select the text, even with just 2 paras, and go back to options and find DO NOT CHECK has appeared again I check in Language/Set Proofing Language drop down .

Even after deselecting again, having selected all document text, when I return to content, errors still there (not highlighted) , even if trying F7 manually :(

Could you share an example document which illustrates the issue? Upload the document to OneDrive or Dropbox and post a "share" link in this thread.