If you check your sidecar files (.dop and .xmp), you’ll find that DPL5 writes keywords to its .dop sidecars too - have a look at the screenshot above!
Before DPL5, Keywords were not transportable to newer versions of DPL except through the database. Now, keywords travel in .dop sidecars too…they just don’t reach their target, which in this case, is a dot update to DPL5.
As far as I am concerned, I expect DPL to be able to update its database with whatever information comes along in the files. As I’ve seen, this is not (yet) the case. Maybe it’s only a matter of checking a version number or timestamp, maybe it’s something else, whatever the cause, DxO should fix it.