This issue affects all rail types.  In most circumstances, pistons can move rails without issue.  However, in some circumstances, notably when rails are sitting on an upward-facing pistons, a rail will break off into item form.

Here's an easy scenario to replicate:

Two rails are connected, one of them sitting on an upward-facing sticky piston, the other rail on any block.  Both rails are flat and are at the same height.  When the piston is powered, the rail sitting above it gets pushed up one block, then the connected rail converts to a sloped rail.  When the piston is depowered, the higher rail is pulled down, and the other converts back to a flat rail.  The rails remain intact when the piston is powered and depowered.

However, if the rail (the one not above the piston) receives a block update while the other rail is retracting, it breaks off into item form.  Redstone powering or depowering nearby, even two blocks away, can update the rail at the wrong time and cause this issue.  Attached pictures show this.

In my experience, this may or may not happen depending on the coordinates or orientation.  Most likely because redstone sends block updates differently depending on coordinates.  The above example appears to happen regardless of location or orientation.

The other time I see this happen, is when a sloped rail is pulled down by a sticky piston, and there is no rail available for the top to attach to.  Rather than convert to a flat rail, it breaks into item form.