Sorry for my late reply but I had to do some testing myself about this.
My conclusion is this:
When in Lazarus you create new form which inherits from existing one all Lazarus does is make New form with same components and connects their event handlers to the same methods that are used by parent form and its components. This information is sotred in *.lfm file of you newly created form.
But as soon as you do any action whoch would result in creating new even handler method (doubl clicking on Button for instance) Lazarus conect apropriate event handler to newly created method in new form's unit. So the information about it being inherited is lost.

When I try same thing in Delphi it automatically adds inherited clause on the beggining on new event method whenever creating any event methods on the new inherited form. This preserves the information about form or component being inherited.
I gues you will have to do this manually in Lazarus until it gets corrected by Lazarus development team.