It is very common. The Suunto I bought in 1999 did it.
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I find it a useful feature, but I always feel weird when doing a short deco... It gives me a 3-4 minute stop.. but then NO safety stop count down..(unless that is included in the required deco time?) Maybe the OP should look for a new instructor and I should look for my manual (after 3 years of use?)
This is not explicitly covered in the manual
It is, however, explicitly covered in your tech training (without which, you should not be doing decompression dives).