Drain-plug gaskets that are too hard tend to come loose. Toyota's fiber washers were too hard at one point and they came loose. These days, they use aluminum core inside and soft fiber outside (unlike the old hard fiber throughout the washer in the past), which is the ideal combination. It's for 14-mm-wrench drain plugs. They don't loosen or leak. You can buy them over the Internet. Toyota part number is 90430-12031.
The old hard-fiber gaskets that got loose:
The new and improved aluminum-core, soft-fiber coating gaskets:
The old hard-fiber gaskets that got loose:
The new and improved aluminum-core, soft-fiber coating gaskets: