Did you install in slot 1 openvix-5.3.039.release-zgemmah7_recovery_emmc.zip or the multiboot image openvix-5.3.039.release-zgemmah7_usb.zip ?
You said you flash all of the images with usb, the only image that need to be extracted to usb are:
• openvix-5.3.039.release-zgemmah7_recovery_emmc.zip
• openatv-6.4-zgemmah7-20200730_recovery_emmc.zip
• openpli-7.3-release-h7-20200727_usb.zip
Once one of these is flashed in slot 1 you should be able to install multiboot images and see them in boot menu..
If not then something wrong with the multiboot image.I put it in the flash ( tested )
1-openvix-5.3.039.release-zgemmah7_recovery_emmc.zip
2-openatv-6.4-zgemmah7-20200730_recovery_emmc.zip
3-zgemmah7-PurE2-6.5-20200729_recovery_emmc.zip
With the first it is possible to insert OpenPli into the multiboot ( 7.2 work normaly,7.3 does not see the slot)
with the other 2 image it is not possible to insert an OpenPli into the multiboot ,because multiboot does not recognize any OpenPli image ( 7.2 and 7.3 )
So the stories about the standard multiboot are not true...
OpenPli in Zgemma H7S can only work in flash or in a multiboot if in a flash OpenVix 5.3.039 but only OpenPli 7.2...
After inserting the image into the flash i inserted the image into the slots...
I just test the multiboot on the Zgemma H7 and what seems strange to me is that you succeed in falshing _recovery_emmc.zip into different slot.
This is not possible with OpenPLI, you need to flash the _usb.zip version.
So here is how I proceed I flash the OpenPLI Release 7.3 USB version normally into the flash of the STB (not in multiboot).
Then I put an USB stick format it from the STB.
Then I create on it a folder called: downloaded_images
In this folder I transfer by FTP the file: openatv-6.4-zgemmah7-20200730_usb
I went into the Flash Manager there in the list of the download_images I have the OpenATV one select it and I flash it successfully into Slot 3.
I boot into OpenATV perform the setup go into the Image selector select to boot back into OpenPLI 7.3 and it is working fine.
There I select to boot again in OpenATV and it is working.
So Multiboot in OpenPLi is working fine if you flash the right image.
Please try try to install PurE2 6.5 and OpenVix 5.3.039..You just tried OpenAtv 6.4 and OpenPli 7.3