Can think of how one would tear their gluteus medius on training for triathlon unless getting charged by a run-away bull or landing on a post

That said it sounds like perhaps this is a lumbar spine issue (previous QL issue) and mayabe the pelvic in relation to that. Of course a long 15% climb, in the saddle?, could have irritated the hip joint/bursae in the area. You need to go training, including hills and work out functionally what is the major aggravation. This might not be know until the day or so after training. Then work backwards to eliminate things.

The pain resolution from your last treatment is impt to analyse with your therapist, especially in relation to any Wikipedia reference-linkMRI findings. On that not make sure they look closely across the hip joint and if you can have a functional MRI then have it in the position of most discomfort.

As for a QL problem: well that might have been tight or in spasm but likely due to something it did to the underlying joints of the spine and ribs and that the treatment to those areas resolved that issue. QL is a pretty darn strong muscle that wouldn't itself be injured with coughing. Again this points to some underlying spinal issues with muscle imbalance and perhaps weakness.

Regardless of what they find on MRI, the cause needs to be assessed and your mechanics and training regimen and even bike setup should be modified slightly as required. Also think about adjusting the bike setup for courses with more hills vs those on the flats.

Let us know the results of the MRI. Best of luck