Natural spawned spider with potion effect lost effect after a period of time

I was playing in hard mode and came across an invisible spider. I caught it and nametagged it to prevent it from despawning. After a while I checked on where I had left the spider and it had lost the invisibility effect. I looked at the wiki and it said the effect had "endless length". While other less reliable sources either said endless or max duration effect length. I don't think it was intended for the spider to loose its spawned buff so I marked it down as a bug.