I came across a similar problem. Using Launch ready ordnance the mine wasn't visible on the map.
The following buoy stayed in position:
Missile Size: 5.995 MSP (14.9875 Tons) Warhead: 0 Radiation Damage: 0 Manoeuvre Rating: 10
Speed: 0 km/s Fuel: 0 Flight Time: 1 seconds Range: 0 km
Active Sensor Strength: 0.49 EM Sensitivity Modifier: 14
Resolution: 150 Maximum Range vs 7500 ton object (or larger): 7 851 459 km
Thermal Sensor Strength: 1.4 Detect Sig Strength 1000: 9 354 143 km
EM Sensor Strength: 1.4 Detect Sig Strength 1000: 9 354 143 km
Cost Per Missile: 5.264 Development Cost: 526
Chance to Hit: 1k km/s 0% 3k km/s 0% 5k km/s 0% 10k km/s 0%
The following mine didn't stay in position and vanished:
Missile Size: 5.999 MSP (14.9975 Tons) Warhead: 0 Radiation Damage: 0 Manoeuvre Rating: 10
Speed: 0 km/s Fuel: 1 250 1st Stage Flight Time: 1 seconds 1st Stage Range: 0k km
2nd Stage Flight Time: 3 minutes 2nd Stage Range: 8.21m km
Active Sensor Strength: 0.82 EM Sensitivity Modifier: 14
Resolution: 150 Maximum Range vs 7500 ton object (or larger): 10 156 855 km
Cost Per Missile: 11.562 Development Cost: 1 156
Second Stage: Martel I ASM - D3 - S52 - R8 - CTH52 x5
Second Stage Separation Range: 1 000 000 km
Chance to Hit: 1k km/s 0% 3k km/s 0% 5k km/s 0% 10k km/s 0%
I've redone the mine with a passive thermal sensor same result:
Missile Size: 6.000 MSP (15.0000 Tons) Warhead: 0 Radiation Damage: 0 Manoeuvre Rating: 10
Speed: 0 km/s Fuel: 1 250 1st Stage Flight Time: 1 seconds 1st Stage Range: 0k km
2nd Stage Flight Time: 3 minutes 2nd Stage Range: 8.21m km
Active Sensor Strength: 0.82 EM Sensitivity Modifier: 14
Resolution: 150 Maximum Range vs 7500 ton object (or larger): 10 156 855 km
Cost Per Missile: 11.562 Development Cost: 1 156
Second Stage: Martel I ASM - D3 - S52 - R8 - CTH52 x5
Second Stage Separation Range: 1 000 000 km
Chance to Hit: 1k km/s 0% 3k km/s 0% 5k km/s 0% 10k km/s 0%
Made a third mine with thermal and emission passive sensors same result:
Missile Size: 6.00 MSP (15.000 Tons) Warhead: 0 Radiation Damage: 0 Manoeuvre Rating: 10
Speed: 0 km/s Fuel: 1 250 1st Stage Flight Time: 1 seconds 1st Stage Range: 0k km
2nd Stage Flight Time: 3 minutes 2nd Stage Range: 8.21m km
Active Sensor Strength: 0.66 EM Sensitivity Modifier: 14
Resolution: 150 Maximum Range vs 7500 ton object (or larger): 9 112 222 km
Cost Per Missile: 11.306 Development Cost: 1 131
Second Stage: Martel I ASM - D3 - S52 - R8 - CTH52 x5
Second Stage Separation Range: 1 000 000 km
Chance to Hit: 1k km/s 0% 3k km/s 0% 5k km/s 0% 10k km/s 0%
Fourth try without second stage this time the 'mine' stayed in place:
Missile Size: 1 MSP (2.5 Tons) Warhead: 0 Radiation Damage: 0 Manoeuvre Rating: 10
Speed: 0 km/s Fuel: 0 Flight Time: 1 seconds Range: 0 km
Active Sensor Strength: 0.82 EM Sensitivity Modifier: 14
Resolution: 150 Maximum Range vs 7500 ton object (or larger): 10 156 855 km
Cost Per Missile: 1.312 Development Cost: 131
Chance to Hit: 1k km/s 0% 3k km/s 0% 5k km/s 0% 10k km/s 0%
My conclusions:
From the combinations above the only ones that stays in place are the buoy, without a second stage.
Don't know if this is WAI.