Same problem with my '16 FLSTFBS. Dealer replaced the first dipstick under warranty. I get that seal wear out, but that was pretty quick.

 

While adding an O-ring to the cap is an old, reliable remedy for a worn/weak cap seal...

If the O-ring or new cap don't work, you MAY have an issue with blow-by... there shouldn't be too much pressure in the oil tank.

If an O-ring and a new cap didn't work for me, I'd first do a compression test. If that was all good, I'd inspect the crank case breather system. Pressure getting by the rings, or not properly vented due to a clogged/broken breather assembly, could allow for too much pressure to build up in the crankcase and find its way to your oil tank....

 

I have this issue too with the temp dipstick.
I find that its me not wiping the rubber plug off before reinserting it into the bung. (pun intended)
I just wipe the little bit oil off the tank.