Confirmed in WaveLab Pro 10.0.3 and 10.0.4, macOS 10.14. When changing the “Time Format” from “Show Units” to “Compact”, the “Silence Duration” input box displays in milliseconds, but the input is in frames. (It took quite a while to figure out what was going on!)
Find one more: when the duration is set to 00:00:00:00 manually and you attempt to use the arrows to increase the duration, the dialog starts “running away” and won’t stop with any repeatable input steps.