Dunno if this was the case with yours, but if If the IAF is only an approach fix and not an enroute fix, it's common for ATC for a different sector, even an adjoining one, to be clueless about it. I don't generally file to an IAF but I have had the experience of asking an adjoining center for direct to an IAF like and received a "where is that?" in response.I have put in the IAF as the fix before my destination airport in an attempt to "plan" the approach from there. However, center had no idea what the IAF was for the broadcast approach and cleared my direct to destination, then when I contacted approach, was told to go back to the IAF fix. So, double the work.
More often than not, the conversation moves forward with me telling them it's the iIAF for the approach I want, they asking me my on-course heading to it, and then them clearing me direct. It's not that much different than when ATC asks for your on-course heading to anywhere they don't know, including the destination.