not all TSSCPP are produced

[mantepse]

Steps To Reproduce

sage: B = PlanePartitions([6,6,6], symmetry="TSSCPP")
sage: list(B)
[Plane partition [[6, 6, 6, 3, 3, 3], [6, 6, 6, 3, 3, 3], [6, 6, 6, 3, 3, 3], [3, 3, 3], [3, 3, 3], [3, 3, 3]],
 Plane partition [[6, 6, 6, 5, 5, 3], [6, 5, 5, 3, 3, 1], [6, 5, 5, 3, 3, 1], [5, 3, 3, 1, 1], [5, 3, 3, 1, 1], [3, 1, 1]],
 Plane partition [[6, 6, 6, 3, 3, 3], [6, 6, 6, 3, 3, 3], [6, 6, 6, 3, 3, 3], [3, 3, 3], [3, 3, 3], [3, 3, 3]],
 Plane partition [[6, 6, 6, 4, 3, 3], [6, 6, 6, 4, 3, 3], [6, 6, 4, 3, 2, 2], [4, 4, 3, 2], [3, 3, 2], [3, 3, 2]],
 Plane partition [[6, 6, 6, 5, 5, 3], [6, 5, 5, 4, 3, 1], [6, 5, 4, 3, 2, 1], [5, 4, 3, 2, 1], [5, 3, 2, 1, 1], [3, 1, 1]],
 Plane partition [[6, 6, 6, 4, 3, 3], [6, 6, 6, 4, 3, 3], [6, 6, 4, 3, 2, 2], [4, 4, 3, 2], [3, 3, 2], [3, 3, 2]],
 Plane partition [[6, 6, 6, 4, 3, 3], [6, 6, 6, 3, 3, 3], [6, 6, 5, 3, 3, 2], [4, 3, 3, 1], [3, 3, 3], [3, 3, 2]]]
sage: B[0] == B[2]
True

Expected Behavior

There should be seven distinct plane partitions in a box with side length 6.

Actual Behavior

Some plane partitions are missing, others are duplicate.

Additional Information

No response

Environment

irrelevant.

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide