Actually this just happened to me as well, I was also curious about whether or not there was a cap, I wanted to try out the third grenade type for the Warlock. The game just began saying "no progress made" on skills after Crucible matches.
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Did you do the new Iron one? Cause I did too and I think it might be from that, if it is at all.
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The Iron Banner? Perhaps, that may be the cause if there is no set cap because that was the last thing I did before I stopped gaining xp.