This is a feature of the game. Spiders have a chance to spawn with potions effects. This was taken from the wiki:
On hard difficulty, spiders occasionally spawn with status effects. For each pack spawn, there is a (10×clamped regional difficulty)% chance of the game applying a status effect. This does not apply to cave spiders. If it decides to do so, then it selects the effect. The spider can spawn with following effects:
Brian Wasserbauer
This is a feature of the game. Spiders have a chance to spawn with potions effects. This was taken from the wiki:
On hard difficulty, spiders occasionally spawn with status effects. For each pack spawn, there is a (10×clamped regional difficulty)% chance of the game applying a status effect. This does not apply to cave spiders. If it decides to do so, then it selects the effect. The spider can spawn with following effects:
Speed (40% chance)
Strength (20% chance)
Regeneration (20% chance)
Invisibility (20% chance)