My X Trail won't go into 6th automatically below 70mph unless I am on a falling gradient. Even if I put it in manual mode, as soon as the speed drops much below 60 it goes into 5th. I don't know whether that is normal, but it doesn't seem to cause any problems, just seems like the car is very high geared. Even cruising at 70 unless I am on a motorway with a slight downhill it won't go into 6th unless I manually select it. 70mph in 5th is around 2,500 rpm. In 6th it's around 2,000 rpm or just under. The car will go into 6th automatically, but only at nearly 80mph or when going slightly downhill at 70. I didn't even notice it at first as the Volvo V70 diesel automatic I used to drive did 70mph at around 2,500 rpm in top gear. It was only when one day on a motorway going slightly down hill at just over 70, I noticed the X Trail change up! The car drives beautifully otherwise, so I don't know whether this is just a quirk or perfectly normal. Still getting used to the car.

Yes, it is normal.
The 6th gear is very long and the rpms are fallig too low. That produces shoot and vibrations. The ECM protects the engine.

SERVICE DATA AND SPECIFICATIONS (SDS)
General Specification INFOID:0000000001303902
*: Refer to MA-22, "Fluids and Lubricants".
Vehicle Speed at Which Gear Shifting Occurs INFOID:0000000001303903
Unit: km/h (MPH)
At half throttle, the accelerator opening is 50% of the full opening.
Applied model
M9R
4WD
Automatic transmission model RE6F01A
Transmission model code number 1XN0A
Stall torque ratio 1.75
Transmission gear ratio
1st 4.199
2nd 2.405
3rd 1.583
4th 1.161
5th 0.855
6th 0.685
Reverse 3.457
Recommended fluid Genuine NISSAN Matic J ATF*
Fluid capacity 7.5 liter (6-5/8 Imp qt)
CAUTION:
• Use only Genuine NISSAN Matic J ATF. Do not mix with other fluid.
• Using ATF other than Genuine NISSAN Matic J ATF will cause deterioration driveability and A/T durability, and may damage
the A/T, which is not covered by the warranty.
Gear position
Throttle position
Half throttle Full throttle
UP
D1→D2 19 – 23 (11 – 14) 31 – 35 (19 – 21)
D2→D3 34 – 40 (21 – 24) 54 – 60 (33 – 37)
D3→D4 60 – 68 (37 – 42) 87 – 95 (54 – 59)
D4→D5 80 – 88 (49 – 54) 115 – 125 (71 – 77)
D5→D6 121 – 129 (75 – 80) 156 – 166 (96 – 103)
DOWN
D6→D5 81 – 89 (50 – 55) 145 – 155 (90 – 96)
D5→D4 56 – 64 (34 – 39) 108 – 118 (67 – 73)
D4→D3 39 – 45 (24 – 27) 76 – 84 (47 – 52)
D3→D2 21 – 25 (13 – 15) 45 – 51 (27 – 31)
D2→D1 8 – 12 (4 – 7) 8 – 12 (4 – 7)

Thanks for that Ahto42. I thought that was probably the case as the car drives too well for it to be a fault. I have had the car just on a year now, but haven't had all that much use from it this year thanks to Covid. As I am over 70 I didn't go out of the house for several weeks and the car just stood outside. This is a car I am planning to keep for a long time, so I just hope I don't get any major faults with it as I am no longer able to do as much work on cars as I once did.