No, that's not a problem of LSH3 V5.1 . It could happen in every "super-MOD" and even in Stock-SH3.
When the convoy leader was sunk, the convoy doesn't know what to do. In LSH3 V5.1 you have many scripted AI-subs and also the "Luftwaffe", who attacks convoys.
So feel happy to meet such a confusion and sink them all.
Greetings
rowi58