Given no one seems to be interested in working on the applet (boo) I decided to tackle the bug myself. Apparently the fact it worked previously was the result of happenstance.
The systemd manager processes only send out signals after at least one process told them to do so. It seems that some process did that on my computer before but no more. From the doc:
Subscribe() enables most bus signals to be sent out. Clients which are interested in signals need to call this function. Signals are only sent out if at least one client invoked this function. Unsubscribe() undoes the signal subscription that Subscribe() implements. It is not necessary to invoke Unsubscribe() as clients are tracked. Signals are no longer sent out as soon as all clients which previously asked for Subscribe() either closed the bus connection or invoked Unsubscribe().
After adding the call now the applet notices state changes for units on the session manager too.
Next step is proper licensing. Getting it reuse compatible.
Then moving it over to kde infrastructure.