The class of Line concerned with Tempo changes (incorporating the text rit./rall./accel.) do not, when the text is individually selected, respond vertically to "Reset Position". (They might respond slightly horizontally). This makes it difficult to get Tempo Lines to snap into the Magnetic Layout grid, or restore them to their default position after having moved them.
Tempo Lines can have their vertical position reset only by first System-Passage selecting (purple box) the bar to which the Tempo Line is attached: but then the "Reset Position" will apply to all objects attached to the bar.
Edit 2015 May 4: another user has commented below that the Tempo Lines will "Reset Position" if the extended line is selected, rather than the word or symbol that starts the line. This is a good work-around for those that know it, but it is very obscure, non-intuitive, and undocumented. So this bug report remains a valid issue.