By setting the soft planning limit of 12 hours, with an allowable extension when delays occur, all this chaos is avoided by rostering 12 hours shifts to the planning limit, and then just putting pressure on crews to extend when delayed…….
Pre March 2020, one major airline was doing just that. Unfortunately though, some crews were convinced into extending subsequent tours of duty on the basis that the planning limits only applied up to the commencement of the trip, not individual tours of duty. When this was raised by line pilots during consultative meetings, the CASA representatives remained mute - thus approving of the procedure (in the eyes of the operator) by virtue of not disapproving of the procedure.
I'd be very surprised if it's not still occurring.
When you extend, you are basically putting all the responsibility on yourself by saying that you are in a fit state to continue beyond the stated limits, which lets face it, are not conservative and already in industries favour.
Which arguably applies at any time whether you are about to commence a duty or are already on duty. Anytime you feel unfit to operate, for any reason, you must cease operating at the first available opportunity. Easier said than done in some companies I know.