I have create an map with 2 command block with the command /title : one with /title (for an title) and an other command block with /title (for an subtitle).
I have destruct the command block with /title (with subtitle) but the subtitle appears still on the screen even if I break the command block (with subtitle)

to create the bug, you need place tow command block with an redstone (the title in first and the subtitle in second as my first screenshoot; and you just need to delete the command block with the subtitle !