I too suffered this issue, thinking I soft locked myself by using the deep loaded blunderbuss to shoot myself up. I restarted the level, having experiencing softlocking on an earlier map.

Keep following the canyon, there's a cliff you need to go up but it's more than manageable with the double jump. You just need to make a turn.

I only found out about it when I jumped in the huge ♥♥♥♥ off warthog which gave me a higher perspective to see the cliff and where to continue.

Also to add, the specific spawn trigger for mcmuffin 3 is to enter a banshee, there are two banshees further down the canyon.