Yes, there are many possible ways, and even many widely used ways, to state an instruction for investment allocations.
And yes, it’s not unusual for a regime to align instructions for accumulated balances with instructions for ongoing contributions.
From context, I’m guessing your query is about a participant’s investment direction expressed as a standing instruction—one that regularly and periodically continues, rather than an instruction that’s one-time or episodic.
While it might be useful to consider recordkeepers’ methods, a more immediate question is whether the allocations the particular recordkeeper’s operations produce follow the text of the form the directing participant signed.
If the allocations a recordkeeper produces vary from those that would result by following the standing-instruction form, it’s time to rewrite the form to communicate accurately and fairly what the recordkeeper really does.
Or if the allocations follow the standing-instruction form but are not what a directing participant expected, it’s time to rewrite the form to communicate helpfully to a reasonable reader.
(We know either effort will partially fail because of some participants’ aliteracy. But that doesn’t excuse trying to write a text a reasonable reader could comprehend.)
If the clients you mention are employers that serve as plans’ administrators, you might be on to something to suggest a fiduciary attend to this.
In my experiences, what recordkeepers do often makes good sense, but sometimes is communicated less skillfully than one might like.