I don't know if this is a real bug, or a bad plugin design. Anyway,
there is a kind of artifact in plugins that buffers data
(jack-rack-1.4.6); easy to reproduce.
For example, when running a delay plugin enabled and processing
audible data is changed to disabled; then re-enabling it with the
input silenced will reproduce the last buffered data, and then returns
to a normal behavior.
I tested this with various delays plugins (Delayorama and others)
with the same result.
This is evident using jack-rack as instrument processor, where the
plugins are turned on and off at differents times in a perfomance.