I've had my Madagascar Palm (Pachypodium lamerei) for around 2 years now and it was doing well the first year and then sometime last summer it started to have issues. Just to note, I had not changed anything. It was putting out new growth but all of the new leaves would start to immediately (or soon after) blacken at their tips, slowly shriveling up the leaf until it fell off (which explains why there's a bald gap in the middle now). The old leaves remained unaffected.
At that time, I brought it back to the plant shop where I had bought it and they inspected it and repotted it as the roots were starting to grow out of the bottom of the planter. They didn't seen any obvious pests but did note that a part of the base of the trunk had some slight give/softness (the roots looked fine according to them though). They also replaced the very aerated mix (which was mostly chunky pieces of bark) with coco coir, which I didn't know anything about at the time but they said it would be good for quick drainage. Based on what I've since read, it actually retains more water and I did notice that it appeared to stay moist for a longer time (correct me if I'm wrong!). Once I realized this, I attempted to combat it by watering less (I went from watering once every week to once every two weeks).
Fast forward to spring/now and the palm has started to put out new growth. The trunk is hard and solid again and for a bit it was doing okay (which is why there are some longer leaves at the the top!) but has started to again show the same issues with the newest leaves. I repotted about a week ago since the roots were again growing out of the bottom and I also changed the coco coir to a much more gritty mix (cactus soil + Jack Bonsai) to ensure proper drainage. Still, despite all of the new growth, all of the new leaves eventually start to darken and go limp. I have tried to look under leaves for pests but still don't see any obvious ones. The only thing to note here is that the shriveled leaves appear to be leaking orange-y droplets of sap (you can see it in some of the pics where they've hardened), there are some faint brown splotches/spots and I have noticed one or two clear hard crystals on some of the youngest growth (see the close-up pic of the new growth--there's a tiny crystal on the top right), along with what appears to be one or two superthin clear threads (not enough for me to call it a web) but again, I don't see any bugs. I also tried the shake-leaf-over-paper trick to see if any spider mites fall off but nothing there either.
I have since continued to give it full direct sunlight in my west-facing window and am watering once a week. Any idea on what's going on here? Overwatering? Underwatering? Pests? Fungal? Something else?
Sorry for the long post but wanted to be thorough. I appreciate any help!
EDIT: See my second post for all photos!