Cleared for an approach, then given a different altitude assignment

Peter R. wrote:

I replied, "Negative traffic" to which he responded, "Maintain
3,000."

Being momentarily confused ...

What's confusing about "maintain 3000"?

Should the controller have canceled my approach clearance first, then

issued the altitude restriction?

His primary responsibility is to keep you spearated from traffic. He
should have given you clear, simple instructions to that end, which he
did.

Would cancelling your approach clearance, assigning an altitude, then
re-vectoring you on the approach have been easier, or clearer, or
safer?

I was initially confused because I still had 5 miles at 3,000 feet
before stepping down to the next altitude as part of the approach ...

Well then, there was no conflict, and no reason to be confused.

... and
it seemed that his first call was simply reinforcing the altitude
minimums on the approach (that is, until he responded in a terse
manner
that he wanted to keep me there without ever rescinding my approach
clearance).

Fortunately, most controllers have enough sense to fill in where more
is needed.