If you are following Chicago Manual of Style guidelines, you should include the % symbol before the hyphen in a suspended compound. Here is the relevant rule on repeating closed-up symbols in numerical ranges, from the sixteenth edition of Chicago (2010):
9.17 Units for repeated quantities. For expressions including two or more quantities, the abbreviation or symbol is repeated if it is closed up if it is closed up to the number but not if it is separated. ...
[Relevant examples:] 35%–50% [and] 3°C–7°C
I note that the suspended hyphen in your example—
a 10- to 15%-a-year increase
—is not standing in for the % symbol after the number 10 in any case: it is standing in for the hyphen following the percentage 10% and is exactly in parallel with the hyphen after 15% in the longer character string "15%-a-year." In other words, if you were to forgo the short form made possible by using the suspended hyphen, the long form of the expression would look like this:
a 10%-a-year to 15%-a-year increase
The suspended hyphen works forward from "10%-" and justifies dropping "a-year" from the long form of expression:
a 10%- to 15%-a-year increase
But there is no logical theory under which Chicago's guideline would support the claim that the suspended hyphen justifies shortening both forward (removing "a-year" after "10%-") and backward (removing the % symbol from "10%-"). Chicago's rule 9.17 indicates that the closed-up % symbol attaches to the preceding number more powerfully than the hyphen does.
If you were interested in doing so, you might make a fairly strong case for dispensing with the suspended hyphen altogether and running the entire expression of the numerical range as a single unbroken string:
a 10%-to-15%-a-year increase
But you don't evince any interest in achieving that result.
With regard to AP style, The Associated Press Stylebook and Briefing on Media Law (2002) seems to hew closer to Chicago than to the advice given to you by the anonymous AP correspondent who wrote to you.
Here is AP's entry for percentages:
percentages Use figures: 1 percent, 2.5 percent (use decimals, not fractions), 10 percent. ...
Repeat percent with each individual figure: He said 10 percent to 30 percent of the electorate may not vote.
So even though AP advises spelling out percent, it explicitly insists on repeating the unit of measure (percent) after each number being discussed as a percentage. From this it would seem to follow that AP would endorse
a 10-percent- to 15-percent-a-year increase
or, less awkwardly, as the % symbol has become more acceptable in main text use,
a 10%- to 15%-a-year increase
It seems a bit odd to conclude that the AP spokesperson who communicated with you was mistaken about AP style—but that's the conclusion I reach after looking at the relevant coverage in the (admittedly not current) AP Stylebook that I have at hand.
