The structdump endpoint changed the way it displays the task config. This change made some thrift internal metadata visible, such as __isset_bitfield on optionals fields and appending underscores on the names of other fields.
For example, here is a fragment of a 0.19 structdump output:
And here is the same fragment, but with 0.18 scheduler:
As the structdump is the only way to get the full configuration of a task through the UI, it is inconvenient to have those fields and naming messing around.
The issue seems to be caused by a change in the way data taken from thrift is converted to be pretty printed; by changing a thrift specific converter to a GSON serializer. The proposed fix is to backport the old converter and restore the old structdump style.