The capability of the transducer for sonar, depends upon whether it also has a (big, black, squared-off) CHIRP sonar module in the circuit. (Otherwise known as a Lowrance SonarHub, or similar device/box.)
Actually, on further research, it may be this GPSMAP doesn't need a 'black box' to get CHIRP sonar, with this transducer.
The only question in my mind is, does a Lowrance transducer work well with a Garmin device? In this case there's no reason to believe it *doesn't* - yet - since you do get some readings of some sort even now.
It's more likely that the connection has become slightly dirtied. Corrosion is a constant enemy of electronics and metals in any maritime or water-going context, especially in salt-water or brackish waters.
Trace the connection from the Garmin to the transducer (transom-mounted). See if the wires/cables are at all damaged, severely bent, or even slightly cut, anywhere along the entire length. (And see whether there's a 'black box' involved, en-route.)
Power Off the system.
Then disconnect the connections to the transducer from the Garmin, one at a time only! and scrub both sides of each connection with a clean dry brush for 30 seconds or so, then wipe down both sides with a clean dry rag cloth until the metal shines, and re-connect firmly, then on to the next connector, and proceed similarly.
(Also check the transom mount of the Airmar transducer.)
Then power On the system, and try it again.