AdoDB gets updated automagically to version 8.4 when DBAL 7.6 is installed and TYPO3 CMS was installed the old fashioned way
on TYPO3 7.6 installations installed the old way (with wget and tar xzf) there is a problem when aktivating AdoDB and DBAL. After installing AdoDb every thing looks good. But after installing DBAL AdoDB gets updated to version 8.4. In the extension directory there is suddenly a local install of adodb 8.4.
Could it be that some composer statements ignore the TYPO3 version if TYPO3 is not installed via composer?