No it's not normal, as you probably suspect it's because it's failing to make contact, this can only be for one of 2 reasons, either the ear socket is slightly faulty, or more likely it's because the pioneer earbuds jack is not mating properly..possibly because it's not the correct spec jack plug. (slight shape/length difference or contact placement) If the socket works okay with other headphones, then obviously it's a issue just with those particular earbuds, in which case I would return them, as sometimes manufacturer tolerance may not be quite right. Especially so where these multi function sockets are concerned.
I've had this happen myself a couple times with some of my wired headphones on the iPad, usually a good clean of the jackplug itself has been sufficient to make it work. or preferably stick with the genuine Apple earbuds, which are designed for that socket, they use finer contact area's, as besides stereo sound they also have the volume control and mic signals incorporated of course.. where the pioneers are likely just generic stereo earbuds..IOW the contact spacing on the jack plug is slightly different.
In theory all jackplugs should work okay, but I know from experience that this is not always the case, for some silly reason you will often see tiny subtle differences with some different makes of earbuds, and on these multi functional sockets some can be problematic.
ONLY DO THIS IF YOU DEFINATELY SUSPECT THE SOCKET ITSELF.
The proper fix is obviously to have the earphone socket replaced, but the cost would be very high indeed to have this done on a iPad out of warranty. The small spring contacts inside the socket can with time lose their tension, a common fault especially where sound can be heard through the headphones but not through the speakers on a portable radio is nearly always caused by a contact in the ear socket not returning sufficiently to make contact when the jack is withdrawn, and often requires the socket replacing. However, it's always worth giving the socket contacts a clean, and the best way to do that is by using the follow technique. IME it works 95% of the time.
On a well used socket one can try cleaning the contacts with something like a piece of softish round dowelling wood, of same size to the jackplug, and round off the front or taper it so it's the same shape and size as the jackplug. Then make a mark with a pen on the wood matching the length of the jackplug... so you know how far to insert it. Then immerse your wooden jackplug in a solution of isopropyl or similar mild switch cleaner for a few minutes, then shake off the excess and carefully insert it into the ear socket.. rotating it and moving it carefully backwards and forwards. Note: never force it, and make sure there's plenty of taper at the front otherwise you run the risk of bending contacts inside the socket instead of opening them up when it's pushed in.
You will likely see black dirt on it where the contacts touch when you pull it out.. give it a rinse in the isopropyl, and repeat the process a few times, hopefully this will clean the socket contacts and hopefully make things good again. I've used this method many times over the years with iffy ear sockets and it does work.
(Note. Dont use Balsa wood as it's too soft.. last thing you want is to have it break off in the socket.)
