DateTimeDiff returns negative for same-day comparison in Architect

Trying to calculate hours worked in Architect using DateTimeDiff between start and end timestamps. When both times are on the same day, the expression returns a negative number or fails validation entirely, despite the end time being later.

Here’s the expression I’m using:

DateTimeDiff([interaction.start_time], [interaction.end_time], "hours")

Docs say it handles same-day deltas fine. Anyone hit this wall?