Viewport or PSpace/Layout bug?

I have a drawing with few tabs and in one tab only any viewport returns this
list. Note the -3 group list is nil. I never seen this

Viewport is not locked, or off. The group code -3 is nil also for newly
created viewports. On other tabs the returned extended data is normal.

Command: (entget (car (entsel)) '("ACAD"))
((-1 . <Entity name: 4306aef0>) (0 . "VIEWPORT") (330 . <Entity name:
4306ae88>) (5 . "2E46") (100 . "AcDbEntity") (67 . 1) (410 . "2ND FLR A
ELEC")
(8 . "Defpoints") (100 . "AcDbViewport") (10 10.991 14.875 0.0) (40 . 18.0)
(41
.. 16.0) (68 . 2) (69 . 2) (12 8712.0 264.0 0.0) (13 0.0 0.0 0.0) (14 0.5 0.5
0.0) (15 0.5 0.5 0.0) (16 0.0 0.0 1.0) (17 0.0 0.0 0.0) (42 . 50.0) (43 .
0.0)
(44 . 0.0) (45 . 768.0) (50 . 0.0) (51 . 0.0) (72 . 100) (90 . 49152) (281 .
0)
(71 . 1) (74 . 0) (110 0.0 0.0 0.0) (111 1.0 0.0 0.0) (112 0.0 1.0 0.0) (79
..
0) (146 . 0.0) nil)