Problem about rate control of x265

[hanyi]

I use x265 to encode a 720p video with crf+vbv.
The commond as below:
./x265 --input wz_720p.yuv --preset medium --input-res 1280x720 --vbv-maxrate 1400 --vbv-bufsize 2000 --aq-mode 1 --rc-lookahead 16 --keyint 90 --bframes 3 --crf 24 --frame-threads 8 --fps 30 --psnr -o wz_720p.265

I chang the crf from 24 to 30,and I found something strange,when the bitrate achives the maxrate,the lower crf will case lower psnr.
24 1459.5768 40.8106
25 1456.7784 40.7936
26 1456.7792 40.7553
27 1457.0216 40.9836
28 1390.7492 40.9035
29 1233.576 40.2643
30 1093.8648 39.6247
From the result above,we can found the psnr of crf27 is bigger than crf26 while they have similar bitrate.
I have test other seqences,all have this phenomenon. The R-PSNR curve is not monotonic increasing,it will decrease when the bitrate exceeds the maxrate.
I want to know how to solve this problem,when the bitrate achives the maxrate how to make the psnr and bitrate not change with the crf decrease.