Yes, it happens if the date is less than 48 hour ago. (or for dates in the future, until the ‘end of tomorrow’ in your timezone)
You can control it with the calendar= variable
Default
[date=2020-02-04 time=18:00:00 timezone="Europe/London"]
2020-02-04T18:00:00Z
(screenshot for future reference: 
With calendar=off
[date=2020-02-04 time=18:00:00 timezone="Europe/London" calendar=off]
2020-02-04T18:00:00Z
(screenshot for future reference 
)