This happened twice in the last 24 hours to me. I called Apple Support, paid for help trying to find the 35 minute file yesterday only to learn it was no where on my phone or laptop. They suspected a major bug, since the phone was at max capacity-- beyond, actually. So we restored the phone. Today I tested a few short voice memos and had no problems so I used the feature to record an interview that approached the 1 hour mark. when I finished and hit done, the settings went back to 00 and it appears nothing got recorded.
DO NOT USE THIS FEATURE FOR SERIOUS, IMPORTANT RECORDINGS. MAJOR BUG.
APPLE needs to alert it's tech support so users who call with this problem are aprised of the bug and not told to simply restore the phone and continue to record!
BE WARE!