@Matt When the tables where all updated from mediumint(8) to bigint(20) for member ids in a past version, the upgrade skipped this step and only new installation used bigint(20) from that point on.
If I recall correctly it was temporarily skipped because it could have caused issues altering big tables (mainly for big sites) in a minor (?) version. It was supposed to be added to the next major version, but it looks like it was never done until now.
That's also when the code I pointed out in the other topic was most likely added: