So it turns out that appending "?pretty" to URLs also affects the source part of responses from ES as of 2.2, unline in previous versions, where the source part would be returned as-is, and only the outer part of response documents would have been pretty-fied. This solves my original issue then.

Having a projection constant for returning the source document pretty-fied from queries would be an option, but then users could just do that themselves if needed?