When opening a text file with CALC, the file is opened directly with WRITER; in previous versions, a conversion window was opened, and the result was opened CALC.
Depends on the LO module, if any, is open. If you want to open into Calc have a Calc module open with focus when opening the file. Otherwise by default, text file with a .txt extension will open into Writer. A CSV/TSV text file with .csv or .tsv will open with Calc and its import filter.