I am assuming that the raw data presented reflects directly what was received by the ADS-b ground station.
The last two ADS-b transmission do not have any altitude data (it has presumably been set to zero as there is no valid data).
Your assumption isn't valid. Those are not original ADS-B transmissions, they are FlightRadar24's processed data, originating from enthusiast Mode S receivers with software that may also have done some pre-processing of the raw data as sent by the aircraft.
If you have been involved in ADS-B development, you will know that the message format quoted in your post
T[UTC];LAT[°];LONG[°];HDG[°];ALT[ft];SPD[kts];RoC[ft/mn];Squawk;ADS-B feeder;Data Source
is never transmitted by an aircraft in that form. Positional data (ie lat/lon) and velocity (groundspeed and track) are always sent separately.
So you cannot infer from the absence of individual parameters in a FlightRadar24 pseudo-message that the aircraft was sending null values.