I am hooking up a new set of LP gas logs in my wood burning fireplace.
I had the plumber install a gas line to the fireplace when we built the
home 2 years ago and it has a "keyed" shut off valve mounted in the
side wall. To be safe, I installed a secondary 1/2" - 3/8" T-valve
with a flared connector where the line comes inside the fireplace for
quick access. The line inside the firebox originally had a 3" nipple
and a cap, but I replaced this with a 1" nipple to ensuse room for the
gas log appliance. There are are no leaks to the new T valve using the
soapy water test.

Next, I installed a flex line from the T-valve's flare connector to
another flare connector on the logs. When I turned gas on at the new
T-valve I saw a very small trace of bubbling between it's flare
connector and the flex tubing and directly turned the valve off. The
flex tube is as tight as is it will go onto the connector, and I did
not use any pipe compound per instructions on the flex tubing.

Can I simply apply some pipe compound to the threads on the flare
connector? Or, do I need to replace the T-valve and/or tubing:?

The flex tubing came with two 1/2" - 3/8" flared couplers, so I could
possibly replace the flared shut off valve a 1/2" - 1/2" non flared T
valve.