Every writing guide says the same thing. Use active voice. The passive is weak, evasive, bureaucratic.
Mostly this is good advice. In one situation it is actively harmful, and it happens to be the situation where client relationships get damaged.
The two sentences
Your client forgot to send the brief. The project slipped. You need to raise it.
Active: "You didn't send the brief until Thursday, so the build moved."
Passive: "The brief came through on Thursday, so the build moved."
Same fact. The first one has a subject, and the subject is your client, and now they are reading a sentence in which they are the cause of a problem. The natural response to that is defence, and once someone is defending themselves you have stopped talking about the schedule.
The second one has no villain. It is just a thing that happened, in a sentence with no one to argue with.
What is actually going on here
The technical term is agentless passive, and its distinguishing property is not weakness. It is that the doer can be left out.
Active voice requires a subject. English grammar will not let you say who did the thing while also not saying who did the thing. So when the doer is your client and naming them would cost you, the passive is the only construction that gives you the option.
That is precisely why bureaucracies love it, and why it has a bad reputation. "Mistakes were made" is the famous example, a sentence engineered to describe wrongdoing without a wrongdoer.
But look at what people object to in that example. They object to the evasion of accountability by the person who did it. Nobody objects to the grammar. Using the same construction to spare someone else is a different act entirely, and it is generous rather than evasive.
The rule I use
Passive when the mistake is theirs. Active when the mistake is mine.
"The brief came through on Thursday." Passive, because naming them helps nothing.
"I missed the update in your last message." Active, because taking the subject position is the entire point of an apology. "The message was missed" is exactly the evasion the writing guides warn about, and it reads as cowardice, correctly.
This asymmetry is the whole technique. Same tool, opposite direction, depending on who is holding the problem.
Where it earns its keep
Feedback that arrived late. "The feedback came in after the review window" rather than "you sent feedback late."
Files that never appeared. "We are still waiting on the logo files" rather than "you haven't sent the logo files." Note that this one is not even passive, strictly, but it works the same way by making the wait the subject instead of them.
Approvals stuck somewhere. "The invoice seems to have stalled in approvals" is enormously more useful than "your finance team hasn't paid me," because it is true, it is neutral, and it invites the client to go and find out what happened instead of defending their colleagues.
That last one is worth sitting with. Very often the person you are emailing did not personally cause the problem, and an active-voice sentence accidentally accuses them of something a different department did.
Where it fails
Do not use it to hide your own errors. Covered above, but it bears repeating because the temptation is enormous under pressure.
Do not use it for the actual point of the message. If your sentence is a request, keep it active and direct. "Could you send the logo files today" is what you want. Passive requests turn into fog.
And do not use it for genuine, repeated, serious problems. If a client has been late with input three times running and it is now a pattern that needs solving, softening the third instance means the conversation never happens. At that point say it plainly and say it kindly, in active voice, as a thing to fix together. The passive is for one-off friction, not for avoiding a conversation you need to have.
Why this is worth knowing
Most writing advice is about clarity. This one is about something else, which is that a sentence assigns roles as well as meaning, and the role you hand your reader determines what they do next.
Give them the subject position on a problem and they defend. Leave the position empty and they solve.
That is the whole thing. It works because it never sounds like a technique, which is also why nobody teaches it.
One caveat, and it is the same caveat that applies to everything in client writing. How much softening a person needs varies wildly. Some clients find this level of tact slightly slippery and would rather you just said the thing. Others need every edge sanded off. Knowing which one you are writing to is more valuable than any rule about voice, including this one.