No, your sensor is likely just fine. The altitude calculation depends on knowing the barometric pressure at sea level
If you do not set the correct sea level pressure for your location FOR THE CURRENT DAY it will not be able to calculate the altitude accurately
Barometric pressure at sea level changes daily based on the weather!
The BMx280 'saves' the last reading in memory for you to query. Just read twice in a row and toss out the first reading!