Watchface altitude bug?

Hello all,
Both in 4.16 and 5.07 the altitude bugs...I am at 28 altitude on the street, and I live on the 2nd floor (so more than that).
The altimeters is now displaying minus 26! It was calibrated yesterday on the street...
I believe when the watch was new (lower firmware versions) it hadn't this behaviour.
Any hint? I have it on automatic calibration and not nighly.