Me and my team have been trying a Petra's run of the Last Wish lately, so this may only apply to those. So when we go towards Morgeth we use the "kick members before they die of not having jump-and-run capabilities". Regardless of whether we actually kick someone or not, once we all go through the portal, the phase does not start, no "taken strenght" appears and we are unable to place a raid banner. We have been able to start the phase by randomly kicking players out of the team and hoping it would work, because you have no way to know what player will do the trick. This happened every time we tried so far, so it's not a one-time thing.