The /z/ sound at the end of a word

ESL students, whose L1 is Spanish, have a hard time paying attention to the /z/ sound, especially in end position: please, eyes, surprise. Learners assume that what sounds at the end is the /s/ sound.

When the /z/ sound is at the beginning or in mid-position, the are likely to hear it: zero or result.
What’s the reason for this? Is there a phonological concept shedding light on this?