OK, some news :
I've made some tests with a friend having the same Aura HD (N204B, firmware 3.15) : He doesn't have any problem on his Aura.
On mine, the problem is reproducible :
- If a do a long press just after having turned a page, it works well. I access the dictionary with the definition of the word i've selected.
- If a do the same long press, on the same word, more than 5s after having turned a page, the long press is interpreted as a standard press.
Thanks to this forum, i've managed to revert to 3.11, and I've go back to standard behavior.