This is due to Photoshop CC 2018:Character leading broken.Refer the link given.
the workaround for this particular issue is to use the shortcut keys to increase or decrease the line spacing rather than use the cursor.(use option+down arrow key), if you are using mac, for windows (alt+down arrow key).
I hope it helps you!
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The problem has been acknowledged by Adobe.
As you rightly say, it occurs on text that has been transformed. There is a workaround that allows you to enter the leading values :
a. Enter the leading value in the Properties panel.
b. When you hit enter, the value in the properties panel box will change to something different (that is a bug) but the value you entered will now show correctly in the Character panel
c. The value applied to the text will be the same as you entered and now shown in the character panel
Note : The workaround does not work backwards - you must enter in the properties box.
We have released an update for Photoshop which is Photoshop CC (19.0.1). This update includes many (50+) bug fixes for the top customer reported issues in 19.0. To update Photoshop CC to 19.0.1, click "Update" in the Creative Cloud desktop application next to Photoshop CC. Please check this link for more info: Photoshop CC 19.0.1 now available | Photoshop Family Customer Community