Seems to work fine for me — but…
…any time I get unexpected or seemingly inaccurate results from any date and time function, I check to make sure all datetime-related fields are at consistent in their use of GMT.
And, sure enough, if I configure {Date} to use GMT but {Calculated Workday} not to use it, I get the same responses you are. (That is, +1 day results in {Calculated Workday} showing the same day as {Date}, +2 makes {Calculated Workday} the day after {Date}, and so on.)
Note the precise response patterns will depend on (1) your current timezone, (2) {Date}'s day of the week, and [possibly] the current time-of-day.
Edit: Sometimes I find it hard to be certain just how a datetime field is configured, especially when it is also configured not to include a timefield. To be able to toggle GMT on or off, the field must be configured to display a timefield — but seemingly whenever one toggles on a timefield, the ‘use GMT’ toggle shows as off, even when it is actually activated. Whenever I’m uncertain, I’ll toggle GMT on and back off (or off and back on) just to be sure of the setting.
