Activate a section in Front View.
Align View to section.
Draw a line in both sides mode. It draws in the section plane, but only one of its endpoints is editable/snappable.
This could be a bug or a setting issue with the section plane. Try checking your OSNAP settings to ensure endpoints are enabled. Also, confirm that the section plane is active and properly aligned.
@djhg I cannot reproduce this behavior. Check the gif below, just in case you have some different setting: