Yes, you do have a 39t triple ring, so that's not the problem. Possibly there isn't a real problem except your habits.

FDs are different tan RDs in that they operate on the drive side of the chain. That means that they're very sensitive to the chain tension resulting from pedal pressure. As you start to shift, there are two forces in play where the chain engages. The tension is pulling it down onto the teeth, and keeping the chain straight. The FD return spring is trying to flex the chain sideways far enough that a plate will snag the tip of a tooth and cause the chain to climb up to where it can jump off.

FD springs have limited force, so if you tense the chain with pedal pressure, the FD cannot force it sideways and start the process. You have to help by relaxing momentarily until the shift begins, or better yet, until the chain is engaged on the smaller sprocket.

There will be differences in how chains shift from various chainrings based on chain line, since the angle the chain is coming from factors into how much the FD must deflect it to start a shift.

Try anticipating your needs and shifting earlier in climbs while you still have good cadence at fairly low pedal pressure. Otherwise you'll need to take your chances and back off pedal pressure and hope you still have enough momentum to keep rolling through the shift.