Each TBF has one TFI. If you have 4PDCHs in a PSET, then any TBF which is setup on this PSET will be assigned one TFI. What this means is that, anytime one of the PDCHs has data which belongs to a particular PSET, it just inserts the TFI as part of the data and sends it.
I don’t have much info on LDSTFI. The only info I got on it says that it includes IP discards due to preemption. If that is the case, I don’t see how that would affect your DL MULTISLOT. And when you say your DL MULTISLOT is bad, what exactly do you mean? Is it low or high?