TIS B MLAT

Getting a position for a ModeS transponder requires 4 receivers for multilateration as no location is sent. If there are enough receivers with MLAT synchronized in that area and they can all receive the aircraft, it will be shown as MLAT.

Often there are not enough receivers participating in MLAT to receive aircraft at low altitude.

ModeC only gives squawk / altitude and no hex id (https://en.wikipedia.org/wiki/Aviation_transponder_interrogation_modes#ICAO_24-bit_address) While theoretically possible it's not practical for adsbexchange to do MLAT on ModeC, so you'd never see a position for ModeC transponders unless it's via TIS-B.

Now as you might have read up on TIS-B is data by ATC which has secondary radar. Thus this target could be either ModeS or ModeC. Or the ATC system is mistaken and thinks the transponder has no ADS-B which would also mean the target is broadcast as TIS-B.