This might or might not be relevant
Other Diseases of Birds of Paradise
Birds of Paradise can contract other fungal diseases including Botrytis blight and Armillaria root rot. Botrytis blight causes leaves and shoots to discolor, wilt, decay and drop, and buds to rot. Flowers become spotted and discolored. High humidity makes the disease worse. To prevent Botrytis, prune off dying tissue and thin the plant’s canopy for improved air circulation. To further improve air circulation, remove debris and weeds from around the plant. Although normally associated with trees, Armillaria root rot can target Birds of Paradise as well. Leaves become undersized and discolored, eventually dropping off. Clusters of mushrooms may be present at the base of the plant. In time, the plant dies.
And this
Pests
Several insect feed on the leaves, stems and buds of bird-of-paradise, causing discoloration, deformed growth and possibly serious injury to the plant. According to the University of Hawaii, bird-of-paradise does not have many significant insect pests, but common parasites like mealybugs, scales and aphids can cause problems. Scales are a common threat to many ornamental and fruit-producing plant species in the United States. These tiny insects remain stationery on the foliage, stems and bark of their host for long periods of time. Scales and mealybugs both drain sap from the bird-of-paradise plant, causing new growth to turn brown or even wilt and die.
See if you can look at the brown bits with a magnifying glass. I grow them myself. I don't have this problem.