opening .py file in jupyterlab causes that "run the selected cells and advance" button in other .ipynb files disappear

Question

The only thing I did was just open the .py file. I tried reinstalling jupyterlab but it didn't help. Interestingly, I can still run the individual cells by pressing shift + Enter - only the button dissapeared, not the functionality. Python version is 3.9 and regarding jupyterlab:

IPython          : 8.2.0
ipykernel        : 6.12.1
ipywidgets       : not installed
jupyter_client   : 7.2.2
jupyter_core     : 4.9.2
jupyter_server   : 1.16.0
jupyterlab       : 3.3.3
nbclient         : 0.5.13
nbconvert        : 6.4.5
nbformat         : 5.3.0
notebook         : 6.4.10
qtconsole        : not installed
traitlets        : 5.1.1

Edit: I installed ipywidgets and qtconsole, still the same problem.