Just before head removal. Try getting a feel for how tight the head bolts are. If some are pretty loose they may have been too soft for example even though new.

Or good stretch bolts over torqued by a poorly calibrated torque wrench. This before the final partial final turns took them too far. Both senarios are unlikely but only takes a moment to check.

I think you also mentioned you took them to torque and then removed them to install the washers. Taking them to torque could have started the stretch process a little. When you retorqued them the second time after washer installation they may have stretched a little more. Then you applied the final stretches. This may have been a little too much.

I have limited experience but the stretch component must be carefully engineered in material wise. Once you reach a certain point they could continue stretching under working load later if their point of no return was neared. This is when a stretch bolt actually will stretch or elongate easier than before. Or under less pressure. If you define half way installed head bolts before removal to add washers may help. For example. Not up to initial total torque, up to initial total torque, total torque and first freehand partial stretch turn.