No it's not connected to sx first. Obsessing & fixating in itself is not about the sx instinct. All instincts involve obsessing & fixating - they are distorted through the ego fixation & there is a stacking precisely because one is blown out of proportion & one is somewhat ignored (which causes problems).
IMO, if you want to link this to an instinct, then it would be most neatly be connected to the sp instinct. SP-last people seem more inclined to understand things via interaction with others or experience, whereas sp types are more inclined to "equip" themselves with knowledge., especially before venturing into interaction/experience. Most sp descriptions note the sp first types to preoccupied with personal projects & interests & creating environments & asserting autonomy which allow for this to go somewhat undisturbed. SP last types have trouble with autonomously pursuing interests & projects, and perhaps they seek out more collaboration, input, and external motivation.
Whenever sp is out of whack, there's either too much or too little reliance on others (or a pinging between extremes), depending on whether it's the fixation, the blindspot &/or what the core type is. sp-dom 6s rely too much on creating secure connections with others, whereas sp 5s rely too much on maintaining distance from others so as not to be overwhelmed with their "demands". sp last 6s don't worry enough about secure/stable connections, which can sabotage their more people-oriented sx & so drives. sp last 5s are not motivated to hoard & channel energy for sp interests, which can frustrate the setting of boundaries in the sx & so areas (more people-oriented & perhaps more energy "demanding), compounding the feeling of being overwhelmed by the demands of reality.
However, this is way too specific to really link to an instinctual subtype. As you see, there are too many possible ways an instinct can even play out, so that many behaviors & tendencies can be explained with many different stackings & core types.