The bug

Spiders with status effects will pack spawn, so you will often see multiple spiders with the same potion effect in the same area.

Further, this is not the case for spider spawners, as only one of the 1-4 spiders will have the effect. 

Steps to reproduce

1. Play on a world on hard difficulty and a high regional difficulty (~6.00 or so) (preferably superflat)
2. repeatedly use /time add 2000 until it gets dark enough for mobs to spawn
3. You can set up repeating command blocks in spawn chunks with commands such as /kill @e[type=zombie] and /kill @e[type=slime] (if on default superflat) to get rid of the extraneous mobs
4. Wait for spiders to spawn.
5. Find spiders with status effects. If you don't see any use /kill @e[type=spider] manually to refresh them. Invisible ones are easiest to see
6. Notice that there are sometimes 2-4 in a small area all with the same effect, instead of just one

Additional information

This is in an area where the local difficulty is ridiculously high, you must be on hard difficulty to see spiders with status effects, and even then it is a rare occurrence. 

Possibly intended, but was an unannounced change.