Sounds like you do have some headspace and rapid case expansion from the BP is freezing the case to the chamber walls is allowing the primer which has backed out by the amount of the headspace to not be overtaken by the case since it is not following the primer back. Just a guess.

Are you full length sizing. If so take a case you fired with smokeless and back sizing die off a couple of turns and load with Blackpowder. Smokeless is pushing case back against breechblock and reseating your primers while black is not. It is a headspace problem.

One thing to try:
Determine how far out to seat your bullets such that they will reach the leade/throat SNUGLY. 
Load more of the BP such that the bullet when seated 
engrave into the leade/throat .
this should be a good test and should fireform the brass
while tight to the face of the breechblock
Will be interesting if this might reduce or eliminate
the backed out primers.
beltfed/arnie

My suggestion would be to not ignore the headspace problem.

Try what beltfed said, then do not full length size them after you fireform them that way.
If you still feel the need to resize them, back off your sizeing die about  a 16th of an inch from making contact with the shell holder and that should retain the new configuration of the cases after you have loaded and fired them with the bullet seated firmly into the lands.
Good luck. 

Do not ignore the situation 
You got to keep the genie in the bottle😉 the primer pushing out is a sign of the genie (pressure) trying to get out .   black powder is an explosive and smokeless powder is a propellant they behave much differently.                 black powder pressure is achieved very quickly and smokeless generally is a more gradual build but can achieve higher pressure. 
The head space on a rimed ctg. Is the distance between face of the chamber/ barrel  and the breach face less the thickness of the rim of the ctg.
If the primer is protruding from the case you HAVE A HEADSPACE PROBLEM !   
Many black powder guns have a very tight headspace  setting often referd to as tight breaching 

14 thous seems kind of loose to me
good luck and be safe

It does - just set the barrel back.  Or as others have noted, don't FL size those cases, a necksize would also work, together with the bullet seated hard into the rifling so you can barely cllose the block.

I have a Ballard that also pushes primers proud of the case head upon firing. The cartage is also 32-40. Pushing a fire formed case into the chamber and laying a straight edge across the rear of the barrel I have a gap of .012. I am using 4227 powder and Federal LR primers. Upon firing the firing pin drives the case tightly against the front of the rim. The primer fires and the burning powder expands the case to tightly grip the chamber walls. The pressure of the burning gasses push the primer backwards towards the breach block until it is in full contact with it. This slamming against the face of the breach block is very hard on the block and will cause it to wear where the primer contact takes place.
The way I have fixed this problem is to effectively thicken the cartage rims by .012. I have made a die on my lathe out of a piece of scrap steel so a 32-40 case will slide into it and the back of the rim sits flush with the end of the die. With my mill I have cut four places at 90 deg. apart that are .012 deep. The recesses are about 1/8" wide. I clamp the die in my vise and insert a 32-40 brass case and with a 1/8" flat end punch I strike the rim so the brass is driven into the recess .012". This method seems a bit crude but it works for me.

A. J.Palik