Long-term reference picture can't used in 7th Gen nvenc(A4000, GTX2070,...)?

In our application, the first frame will be set as the long-term reference frame, and all other frames will only refer to the long-term reference frame. When I use the 6th Gen nvenc(Tesla V100, GTX1070,P2000…), I set ltrTrustMode=1, it can work well.

But it can’t work on the 7th Gen nvenc. Maybe this mode is deprecated now. So, I set ltrTrustMode=0, and mask the first frame(or other frame) as LTR frame, it still can’t work.

And then, I run the same application on the 6th Gen nvenc. If I mask the 2nd frame (or other frame, except for the first frame) as LTR frame, it can work. But I mask the first frame as LTR frame, it still can’t work on 6th Gen nvenc. Further more, I set gopLength=25, and mask every I frame as LTR frame, all the I frames were not set be LTR frame successfully.

So, I want to know, for mode with ltrTrustMode=0, does I frame can’t be set as LTR frame. and does the 7th Gen nvenc not support LTR frame now?