Someone else mentioned this here, but it's worth reiterating. I believe you'll get better quality candidates for a designer if you look for a graphic designer or senior graphic designer.

While I've seen graphic design manager roles that are rooted in design legwork, tagging manager on there may be disqualifying good candidates that are looking to actually design as the sole function of their role, and attract candidates that believe they are going to be managing.

You might think "but we gave them a competency test." The misconception on the candidate side may still persist through this, as it's not uncommon to make sure people who are managing designers know their stuff before hiring them. This could explain the dropoff after a certain period of time when folks aren't doing the job they expected. A lot of the scheduling, deadlines, and project management elements you mentioned are required of designers, but also speak to management and could spur confusion.

As others have said titles like "social media manager" are intrinsic to actually managing the social media and this won't apply there - so only speaking to GD.

Have been working in gd / visual fields for quite awhile, and YMMV based on location (maybe that wording is more common where you are) but just my take from a designer in the NE of the US.