recently, all my multi-word searches have been
returning additional results matching only the first
word in the query.

e.g. search for "foo bar bat" also returns results
matching only "foo".

it's obviously not due to poorly named files, but
some problem or change to how searches are handled.
until now, it's seemed that there's always been an
implied AND between search terms. has this changed?

i don't think there have been changes like these.

mahalo

The way that Gnutella works is that searches get matched/processed on the receiving side. So "foo bar bat" goes out over the network and it's up to whoever that gets it to decide if and what files they have that match.

For example older (pre-ultrapeer) clients are/were capable of substring matches, so you could search for "foo" and get back a file called "foobar.mp3". But currently with LimeWire (and any others that use the QRP, i.e. connect to ultrapeers), if you search for "foo" you won't get "foobar.mp3" back from any LimeWire clients (or BearShare or Gnucleus), but something like Xolox will return "foobar.mp3".

It's likely that what happened was that your search query got received by a client that threats "foo bar bat" as {"foo" OR "bar" OR "bat"} instead of {"foo" AND "bar" AND "bat"} like most clients and hence you get back "foo.mp3". This behavior is considered acceptable since that's the point of having searches matched on the receiving end in a non-homogeneous network.