Hi @Laura_Dragne , welcome to the forum 
Regarding your issues:
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Formulas are essentially number fields which, for as long as I know, do not support subtasks roll up to their parent task. This is only currently supported/possible with Asana’s native time tracking Estimated field.
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This appears to be a bug. Could you capture it into a bug report, which you can create in the Report a Bug section so it could be better visible and tackled there?