I work with a team that spans native speaker to barely able to form English sentences, and communicating clearly in a mixed audience like that is really a completely separate skill from native language use.
For the ones with minimal English it's often clearer to use phrasing that a native speaker would say is obviously wrong, but matches their native language patterns. Or use simpler but not-quite-appropriate words.
And for the native speakers, the awareness (or self-regulation) to eliminate idioms and less common vocabulary is nigh-impossible for some people.
I find that awareness and avoidance of idioms is much easier once one has learned (well) a second language. I am fluent in Esperanto, and somewhat subconsciously translate much of what I write as I am writing it. It leads me to choose simpler words and grammatical constructs, and to avoid circumlocutions and especially idioms.
For the ones with minimal English it's often clearer to use phrasing that a native speaker would say is obviously wrong, but matches their native language patterns. Or use simpler but not-quite-appropriate words.
And for the native speakers, the awareness (or self-regulation) to eliminate idioms and less common vocabulary is nigh-impossible for some people.