I recommend farming the Snarla chest in WPM for sentient xp, it is my Old FaithfulSaekee,
Think I've seen one drop since u59 but haven't been out there since the patch. At the time I was trying to prove to someone who needs sentient xp that they do drop there so it was kind of a relief. If I find any in the next little while I'll let you know here.
White Plume does like to drop the sentient. I've gotten them outta all the optional chests (snarla, that one behind the secret door in the left tunnel, and the rare that pops sometimes by the mudpit platter jump) as well as the end chest. I have never gotten sentient xp's outta the rares in Saltmarsh, but they have popped in the quests chests from time to time. But take my opinion on htis with a grain of salt, the horrible turtle treatment of the RNG is a running joke among my freinds. It really hates me. LOLI recommend farming the Snarla chest in WPM for sentient xp, it is my Old Faithful