Thank you for using EmEditor Professional!
I reproduced the problem with a large file. Since it takes long time to adjust a large file, EmEditor did not automatically adjust columns when it opens a large file. On the next minor version, EmEditor will look on the first 100000 lines to adjust columns.
Please make sure “Detect TSV” is checked in the File tab of configuration properties. If this is checked for the opened file configuration, EmEditor should always detect a TSV file if a file is properly separated by tabs.
Please let me know if you have further questions. Thank you!