I'm trying to understand your workflow. But it seems backwards. Are you working with raw files? The normal workflow when using Lightroom is to start with Lightroom and then turn to Photoshop when needed for additional editing.
Yes, my workflow begins in LR, and then I export the file as a PSD to continue editing in PS.
I just followed your workflow. In other words I used the "edit in" option to send an image to Photoshop where I made a significant change in the tone curve. Then I saved that image, which returned to Lightroom. The Lightroom tone curve still displayed as a linear curve. In other words it didn't reflect the changes that I made in Photoshop. However the image itself displayed the changes that were made. So apparently saving your PSD in Photoshop will not force Lightroom to display the Photoshop changes. As far as Lightroom is concerned, that is the new linear curve. The only curve adjustments you will see in Lightroom are the ones made using Lightroom.
To summarize what I'm trying to say, whatever you change the curve to in Photoshop becomes the linear curve in Lightroom. Sorry that I am so poor with words sometimes.