in question vs doubtful

in question and doubtful both are adjectives.

Word	Noun	Adjective	Verb	Adverb
in question	No	Yes	No	No
doubtful	No	Yes	No	No

As adjectives, in question and doubtful are synonyms defined as:

Other synonyms of in question include dubious, dubitable.

in question (adjective)	doubtful (adjective)
open to doubt or suspicion	open to doubt or suspicion
	fraught with uncertainty or doubt
	unsettled in mind or opinion