I’m guessing TRIPPLITE-12X::tlCoolingEnvironment is not a table, do you get an error when you run this:
snmptable -v1 -c public 172.16.48.144 TRIPPLITE-12X::tlCoolingEnvironment
Using the [[inputs.snmp.table]] section on a non-table OID isn’t really supported, it will walk the tree but Telegraf doesn’t know the the OID -> Name mapping to name fields unless you specify it like in your second example.
