In the BTU change log (https://minecraft.gamepedia.com/Bedrock_Edition_1.2#Items_2) it is listed that items will now go up a 3x3 glass item elevator as in other versions, however when this is attempted both on (1.2) XB1 and Win 10 with a design that works in 1.12 and TU57, the items escape into the hopper beside them (depicted in the YouTube link below) or onto the floor when no hopper is present (not depicted in video). This function was intended to work as it has been added in updates to several platforms and listed in changelogs as intended. It also does not work on mobile devices.

https://youtu.be/z4B5SKruVBA

It also seems, as shown in the video that if the items are allowed to cycle from the dropper, into the hopper, and back in the hopper with such speed, items are deleted. (I started the demonstration with 64 pistons and ended up with 38, no items reached the top hoppers.)