So I'm preparing for my Plantera boss fight. I made an arena in the underground jungle, but a portion of it becomes the regular underground due to a lack of jungle grass. Will this be a problem during the fight?
Not only does it enrage if you leave the underground jungle, but entering "contaminated" jungle biomes as well. As per the wiki:
Plantera only becomes "enraged" if it follows the player out of the Underground Jungle, whether it be to the surface, to the Underworld, to a corrupted or hallowed part of the Underground Jungle, or to a completely different biome. This causes it to obtain double damage (quadruple in the second form), double defense and increased movement speed, as well as changing its attack patterns and directly rushing the player. Plantera's behavior returns to normal upon re-entry of the Underground Jungle biome.
Simply adding more jungle blocks to the affected part of your arena should solve the problem. Place a bunch of mud and then plant several jungle grass seeds. You should have plenty of materials after creating an arena down there (assuming you dug out a large open space to fight in). I believe lihzahrd temple blocks can also be used to make artificial jungle biomes.