Register choices that survive AI correction

Learners frequently ask whether an AI rewrite preserves the original register when it substitutes "appreciate" for "thanks" in a business email. The underlying rule, drawn from usage guides such as Fowler’s Modern English Usage, requires that the selected word match the social distance and purpose of the exchange; deviation alters the implied relationship between writer and reader. An exception occurs only when both parties have already established an informal baseline through prior exchanges, yet that baseline must be confirmed rather than assumed. How many such substitutions have you seen change the tone of a message without anyone noticing the shift?