Congratulations Romain on hitting your half marathon distance too! We know you have some speedy legs and some good execution to top it off!
That said, it does sound like some of this is coming at a cost to your glut strength/activation. Typically when the hip flexors start to take over, this is a sign that your gluts are not engaging properly. They can still be strong (ie doing clamshells) however they are not engaging when you need them too (with each foot strike).
Thus I would recommend that you scale back on both distance and speed while you focus on retraining your gluts from a neuromuscular standpoint (‘thinking’ about engaging your core with your belly pressure and gluts BEFORE you start each exercise or repetition. If you lose either engagement, stop the exercise, reengage (core, then gluts) and start again. We talked through this during the injury prevention webinar. If you do not do the engagement and maintain it, you are only working the muscular system. Thus you can have strong gluts but when you got to use them again, the injury or compensation returns (like your hip flexors taking over instead of your gluts).
Yes, this is a very different way of thinking than most physical therapists or coaches would give you. However this is the difference between resolving injury or compensations or not. So hopefully this give you a good place to start working!