It is weird
Usually 688's TA can pickup an old generation diesel electric submarine's Narrow band signal >15nm
what's the length of the TA and what's the speed of 688i?
I would guess the speed is too low so the TA dipped below the layer
another explanation is TA is following your submarine's maneuver , it is a curve shape and not straight behind your sub. in that case it will not be able to locate the contact at the same direction from the hull / bow sonar
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