Appreciate the tag man
OP, here is my theory (this is just a narrative that makes sense to me, based on my very incomplete knowledge).
In a lying leg curl, your hip is placed into more extension (i.e. your hip is straighter). However, many of us have limited hip extension due to varying reasons (which include getting stronger). To get around this, the body may dump into anterior pelvic tilt, which can lead to neurologically “shut off” hamstrings. In a seated leg curl, your body does not need to produce APT, and therefore you can effectively drive hamstrings.
Another factor that could be contributing is something called “active insuffiency” which basically describes a situation where a muscle is too contracted to contract any more. In a lying leg curl, the hamstrings are shorter because the hip is in more extension, and therefore have less potential to contract any further. This could also explain the massive weight drop and why you feel the exercise less in your hamstrings.
In terms of actually applying this information, it doesn’t really matter which one you choose. If you can get a great pump with seated leg curls without needing to think much, then I would do seated leg curls.
If you really really really want to do lying leg curls, you’ll probably be well-served to learn how to “stack” your pelvis and ribcage, and feel your hamstrings in greater and greater hip extension. I would start with the 90/90 hip lift (video below) and progress the drill by moving my body further and further away from wherever my feet are, basically letting my hips and knees become straight over time. Remember the goal of this activity is to feel your body in good positions, and feel your abs and hamstrings working (but not dying).