I don't know if it's perhaps a regional thing but, in the UK, "being patronising" is used pretty much exclusively in the pejorative sense, with a similar meaning to "condescending". I don't think I've ever heard (in actual conversation) "being patronising" used to mean someone is giving patronage, in fact - we would say someone is "giving patronage" or "is a patron" instead. We also pronounce "patronise" differently, for whatever reason: "patron" is "pay-trun", "patronage" is "pay-trun-idge" but "patronise" is "pah-trun-ise".
It seems the pejorative use of the word dates back to at least 1755, too, so it's not exactly a new development.