Hikey960 cannot output images to the monitor via HDMI (version: android_8.1.0_r29&master)

I install the Android 8.1.0 (r29) on my Hikey960,the adb shell is work ,but there is no image output in the monitor connected the hikey by HDMI.
On the master branch , the images were output normal In March of this year.but now,it is not work.
Do you know why there is no image output in the above branchs?

Thank you for helping.

It is the same monitor that you previously had working? Hikey960 does not reproduce HDMI timings correctly and some monitors may be too sensitive.

Also, how did you install the newer build? Did you run the install script? Or did you individually “fastboot flash” the partitions? Somewhere around APRIL, there were changes that necessitate firmware updates, which are not performed by fastboot flash system/boot.

Thank you for your prompt reply.
It is the same monitor that I previously used work.
I executed flash-all.sh script to install the newer build.and ,just now,I try to use the new build to install again by the following command.emmm, it not work.then I use MARCH 's master build to only fastboot flash boot.img & system.img,the image is output .

fastboot flash boot out/target/product/hikey960/boot.img
fastboot flash dts out/target/product/hikey960/dt.img
fastboot flash system out/target/product/hikey960/system.img
fastboot flash cache out/target/product/hikey960/cache.img
fastboot flash userdata out/target/product/hikey960/userdata.img

The code for my local master branch has some problems for various reasons.
After re-updating, the phenomenon is normal.
But,Some versions of 8.1 do have problems with image output.
I will continue my work on the master branch.
Thank you for you answer .

I also encountered the same problem, how can I fix it?

If your problem is exactly the same (display worked but not on 8.1.0 r29) then you should be able to switch to master branch instead.