See image (middle entry, taken from summary after session on web): a word that was tested only once (and answered incorrectly) shows up in Difficult Words session.
Presenting a half planted word during a Difficult Words session doesn’t make any sense from a learning perspective. Moreover, as Difficult Words sessions on the web are full-typing and it’s relatively rare (at least for me) to remember an average word after one test, the chance to get it right are about zero - that is why Memrise tests 6 times of course during the planting phase, and rightly so.
Note: this word has three streaks.
The first was during the Planting (=Learn New Words) Session: answered incorrectly.
The second was during a Difficult Words session: again incorrectly.
The thirds during the same Difficult Words session, but now presented as multiple choice: this time answered correctly.
The reason that word wasn’t fully planted is because the planting session was broken off, either due to loss of the internet connection or because the study was interrupted. Neither is a bug; the handling of the result by the server is.