I decided to "go to school" with FOTRS Ultimate v0.62 on a clean JSGME "Compared Snapshot" stock game. Jldjs, you say the Propoise sub does not keep the "Lock" on the target. There are two versions of the torpedo attack....one called "Torpedo Attack EZ", the other "Torpedo Attack Difficult". I ran both, the following images are from the "Difficult" version:
The School test mission has the Porpoise starting at -15 meters, the depth of the Porpoise at Periscope Depth is -17 meters. The above "Depth Dial" shows the needle at periscope depth....56 ft (17meters). I had to strike the "P" key to go to Periscope Depth in order to test things as you stated.
The same approaching Cruiser from the periscope station @ 305 Relative Bearing:
I've got a "lock" on target with it approaching at 12 kts.
I followed it as it passed in front of the sub, some Sampans passed by the scope....still had a good "lock":
I continued to mess around with the game, changed views, went to the conning tower, back to the periscope station, out to the external camera:
....the "lock" stayed with the Cruiser:
The Cruiser is getting out of range now, but the "lock" held throughout the mission.
I don't understand the problem....there isn't any as I see. I suggest you have an issue that's self inflicted. Either with your game having "mod soup", your stock game is corrupted, or with the "Realism" settings you
MAY be using. If you've got your "Realism" settings at a level that changes the behavior of the game....what do you expect to happen?