I don't know if this is a known bug (before patch 8 this work well as can I remember).
The two additional Level 1 Spell Slots (lvl 10 Arcane Trickster) is not given if you have one or two lvl in another class (I tried with Wizard) before lvl 10 (if you star with a Wizard and then multiclass in a Rogue you will never have the additional spells), it only work if you first take all the 10 lvls as a Arcane Trickster and the last two in Wizard.
