To check whether TLSQ Meet the Malfoys triggers, I sped through Lone Wolf and some story chapters...I had one character during year 2 on modded game version 4.3.0, who missed two TLSQs (Lone Wolf and Meet the Malfoys).
To be certain I created a new character with mods on 4.2.1 and I just triggered Lone Wolf.
So my guess is, that for some reason the modded game version 4.3.0 interferes with some of the TLSQs...
Lo and behold the TLSQ triggered at the appropriate point.
Guess I stick to Version 4.2.1 as long as I can since with this version I get what is important to me: content.