I have found that even mason jars leak. I keep mine in closed cabinets and can smell the tobacco when I open the doors, even though I obsessively tighten all the lids to the point where I'm afraid I'll crack the glass. The lakeland and latakia scents seem irrepressible.They're good for perhaps 7 years and then watch out. With a round tin, the seal pressure can be equalized all along it's circumference, but with square and rectangular tins, no way. It's strongest at the corners.
Most of these tins are slowly leaking from the get go. Here's a simple way to prove this. Take a number of your square and rectangular tins and place them in a tupperware container with a tight sealing top and leave them for a few weeks. Put the Virginias together in one container and English/Orientals in a different one. Then pop the top and take a sniff. You'll smell the tobaccos. They're leaking out of the "sealed" tins.
Over time the pressure equalizes and the seal fails. That's one reason that I don't spend money on vintage tobaccos that come in rectangular tins that are older than 10 years. The percentage of failures goes up.
Greg Pease wrote about this and no longer advocates leaving tobaccos to age in these tins, bur recommends immediate transfer to jars for long term aging. He found too many failures in his own cellar.
Mylar bags don't have that problem though.