Saddle gives lower buttock / high thigh pain

Hello guys, I’ve been lent a saddle for test and the title basically says it all. It’s a Fizik Argo Tempo in 160mm size. I’d like to understand if it’s just not the one for me or if there may be a size or fit issue that can be addressed. I just had a bike fit recently, now I fitted the new saddle at the best of my capabilities by respecting the measurements and instructions by the fitter. Front at 0 degrees (the saddle is a bit slippery and at -1 the change in weight distribution towards the hands was material). Will double check again during the weekend though.

Here is some additional information: - 35 yo, 1.94m, 84k, 130-135mm sit bones - aluminium road bike (Cannondale Caad10) with pretty aggressive geometry (quite stretched), 10cm drop. I mainly run on the drops, or 50/50 between drops and hoods - I ride 4-5x a week, usually 1.30h on average (sometimes it’s 1h, sometimes 2 or 2,5), i am quite fit since I used to run 3,500 km a year until I discovered road bikes - i use Gore bibs with a medium chamois and Assos Mille with their top of the line 14mm chamois, depending on ride duration

Today I did a 2h hard ride on the trainer. I kept fidgeting on the saddle moving ever so slightly fore and aft trying to get to know each other better, I don’t know if it’s because I’m unsure where to place my butt on it exactly, or just because of the new sensations, but couldn’t really find “the sweet spot” except for 10-15minutes at a time.

After 40-50 minutes the saddle started to feel quite hard and uncomfortable, and I “felt” it behind my lower buttock / high thighs, not just interfering with my movements but like a bit painful to muscles/tendons in the area. Now still feel a bit sore there, more so on the right leg side than the left one. It seems purely a muscular / tendinous sore feeling, no sciatic nerve issues or such I think. No chafing or skin issues whatsoever.

Sitting more towards the front partly reduced the issue (still better on the left side and worse on the right), but after a while it became a bit uncomfortable on the front part of the perineum (short saddle with a squared off nose tip). After 1.30h I felt better but still had to fidget a few mm here and there every few minutes, at 2 hours I was basically waiting to finish to get off the bike. Not really in pain, just very uncomfortable. Now I still have a little bit of soreness in the topmost back thigh area / lower buttocks remains (still more on the right side), as well as a bit of the same sensation on the narrow bone spurs at the sides of the perineum / testicles (pubic rami?). It’s not painful, it’s just “there”. Very different sensation that after similar rides with my old saddle (Ergon SMC4 Gel in large size, maybe a bit too soft for my weight and that put undue pressure on perineum / back of the testicles)

Could it be a saddle size or shape issue? A fit issue? A buttock positioning on the saddle issue? A chamois issue after a long-ish ride? A habit yet to build to the new saddle? The saddle feels great to the touch and looks awesome, I can test it for other 2 weeks max. Do I have any hope to make it work for me?

Thank you for whatever tip you may have!