Reguardless of what is causing it, it seems common perhaps with Trojans?
I have 6 L16P 6 volts 360amphrs each. Also 4 just a bit smaller. All 10 of those batteries have the raised pos post hump in the plastic. I got those from a scarp yard and 6 have been very good after pusle charging alot to bring them back. 4 still never came back to life yet but still working on them at times.
As long as they work ok I would not give up on them as yet, but I might consider the need for new ones in the future just in case.
I had those working 6 in the back of a pickup truck and was charging at home then dumping to another bank at a different house where I could not put up a mill. Also was charging from the truck while driving etc..
All that bouncing on dirt roads, some deep discharges, fast charging etc.. I am not surprised I had 2 of them go dead on me. They were dragged out of the trash when I got them anyhow. And when those 2 died it was when I droe about 300 miles charging one day, so I might have just burned those 2 out??
So I geuss it may not be good or right that the post has the plastic hump in the case, but my exerience so far is they still work great for a long time anyway and that hump does not particuly matter unless something else is wrong.