I update before going to sleep. In the end there were two challenges that were buggy, ringleader and lordsmach (32K and 35K xp, when it was my first run). Yes, I'm sure it was my first run. This toon is my main and I've definitely made several TRs this year. My BF's toon, who I always do TRs with and is the same level as mine, got full xp for the first time in all the challenges. Those two failed for me, the rest worked well.
I had previously played mabar in another toon, a level 26 sorc, also accompanied by my BF (same level as my toon). No problems here, full xp in all challenges for our two characters.
I had previously played mabar on another level 31 toon (a rogue), without my BF. This was a mule, so I'm not sure I would have played mabar other years with It and I definitely haven't TR it this year. All the challenges gave me xp like the first time, except for the two that failed with my main. I'm not sure I haven't played mabar in other years with this toon, but I doubt it since the rest of the challenges gave me full xp.
There is a bug at play.