WhatsApp is currently having issues with the ‘last seen’ status.
Therefore sometimes the status is not visible. Potentially it’s a bug in the Android app.
We’ve reached out to WhatsApp to address the issue. In a response the company said that it’s investigating the matter. It’s unknown how long it will take before the issue is solved.
Update April 28th: WhatsApp confirms that it rolled out a fixed last Wednesday. In the meantime, users confirm that the status is fixed now. Solved!