Since no one has replied yet, I'll take a stab.
I haven't used Tropica, but it seems all these "soils" share some similarities. ADA Aquasoil lowered my pH to 6 and it sucks out my KH and some GH. From what I have read, this starts to stabilize a little around a month out and improves greatly around month 3.
So I'd say this is normal. Since you don't have animals in your tank yet, I think you are fine.