btGeneric6DofConstraint, btGeneric6DofSpringConstraint, and btGeneric6DofSpring2Constraint are susceptible to gimbal lock

[jrz371]

The btGeneric6DofConstraint, btGeneric6DofSpringConstraint, and btGeneric6DofSpring2Constraint are susceptible to gimbal lock/erratic motion after exceeding +/- 90 degrees on the Y axis.

There's a couple of threads on the Bullet forum about it. Here's one for example.
https://pybullet.org/Bullet/phpBB3/viewtopic.php?t=12159

This is especially an issue in the btGeneric6DofConstraint and btGeneric6DofSpringConstraint since you have no way to adjust axis ordering. On the Spring2Constraint you can set a locked axis as the middle/problematic one and avoid the issue in most scenarios, but can still crop up when you have a wide arc for the middle axis in the ordering. Perhaps applying the patch from issue #72 can help mitigate this.