For recording and display purposes, different from the source ADS-B transmission, FR24 nullifies some of the altitude data.
When an aircraft is geometrically below the 1013.25 isobaric surface, FR24 presentation would need to show negative values since ADS-B transmits level referenced to standard pressure.
Perhaps to avoid confusing the common inernaut, all sub-zero readouts are truncated. When our ilk try to convert ADS-B indicated FL back to ALT, that data is missing.
The cutoff layer is
FL < 0, which is the same as
ALT < (QNH - 1013.25)x27
The modest local 1018 hPa at occurrence time means displayed 0 by FR24 can be any ALT below 125 ft. (XPDR codes in 25 ft increments). Down to ELEV, that is
.