My PC Randolph Gavrilov has used harvested poison from spiders multiple times without incident. Yesterday during the Spiders quest I applied a potent spider poison near the end and realized at some point after battle that the vfx wasn't going away on the sword, and that the on-hit poison damage was now permanent. It remained after resting, re-logging and transitioning. Attempting to apply other poisons fails. I am unsure whether a reset will fix it but I am inclined to think it won't, and I will probably need to contact a DM about having my weapon replaced.
I'm not sure exactly how this happened.
EDIT: Reset has not fixed this.