When the campfire_cosy_smoke particles stack at one point, they start to look transparent and other translucent particles (like ambient_entity_effect, cloud, etc) are not rendered through them. This effect dissapears if there are a lot of other translucent particles close. If the particles are not translucent, such as minecraft:angry_villager or minecraft:crit, the campfire particles will continue to look transparent.

Purple lines also appear where the campfire particles are.