I've had trouble with some of the later transmission drain plugs, too. The late drain plugs have a reduced size hex head, 13mm instead of 22mm, on the same size plug. The problem is worse with the cast aluminum case transmissions where evidently there is a little dissimilar metal corrosion. You must be careful not to tighten the 13mm head drain plugs very tight, or they won't want to come back out in a few years.

When removing one of these drain plugs, you must start out with the correct size (13mm)six point box end wrench. Even six point sockets have a little tendency to tip off the head of the drain plug. Once the 13mm hex head is damaged, you've got big trouble.
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john