Seriously, why would you even cut it down from 300 to 50? This makes no sense and seems to just be to inconvenience players.
Did you happen to look at the pentaxid tanks now and notice that each slot now accepts a full stack of pentaxid, instead of the 1 per slot it allowed before? That's why, I assume.
Stack sizes are also editable in the config.ecf file. You can set pentaxid all the way up 50,000 stack size if you wish. That's the max the game will allow without bugging out.
Well this is only changable if you are the admin of a server. Well here I go bother him again since this seems everything I do since the new update anyway.
Where does 200 come from? There are 8x8 =64 slots in SV's pentaxid tank. 64 x 50 = 3200 pentaxid per tank. You do go over the volume cap but that stuff is disabled by default and noone's going to use it.
Pentaxid Tanks are now Volume restricted, for BA/CV to 1000 Volume, SV/HV 500 Volume - that corresponds to 200 -4x50- for BA/CV and 100 - 2x50 - for SV/HV. Slot numbers are not relevant - might get changed sometimes in the future.