I'm using Sandboxie Plus v0.7.4 (Portable) in Windows 10 21H1. The running processes started in Sandboxie can see and communicate with each other and also with the ones on the Host (running outside of Sandboxie). I've enabled the options "Advanced Options / Hide Processes" and also manually included the processes which should not be seen by Sandboxie programs but that didn't help.
I thought the main idea of Sandboxie is to prevent exactly this behaviour!?
E.g. if I install the Mining Software for Chia (see chia.net) the processes it starts in Sandboxie are not secured from other processes outside of Sandboxie. I'm sure there are other programs with this behaviour but I haven't tried them so can't speak about them.
INI-File of Sandboxie:
[Test]
Enabled=y
ConfigLevel=8
AutoRecover=n
BlockNetworkFiles=y
RecoverFolder=%{374DE290-123F-4565-9164-39C4925E467B}%
RecoverFolder=%Personal%
RecoverFolder=%Desktop%
BorderColor=#00ffff,on,6
Template=SkipHook
Template=FileCopy
Template=qWave
Template=BlockPorts
Template=LingerPrograms
Template=Chrome_Phishing_DirectAccess
Template=Firefox_Phishing_DirectAccess
Template=AutoRecoverIgnore
BoxNameTitle=n
BlockNetParam=y
DropAdminRights=y
FakeAdminRights=y
ClosePrintSpooler=n
OpenPrintSpooler=n
AllowSpoolerPrintToFile=n
CopyLimitKb=81920
PromptForFileMigration=y
CopyLimitSilent=n
NeverDelete=y
AutoDelete=n
AllowRawDiskRead=n
NotifyDirectDiskAccess=n
HideOtherBoxes=y
HideHostProcess=daemon.exe
HideHostProcess=start_timelord.exe
HideHostProcess=start_harvester.exe
HideHostProcess=start_full_node.exe
HideHostProcess=start_farmer.exe
HideHostProcess=start_introducer.exe
HideHostProcess=start_wallet.exe
HideHostProcess=Chia.exe
PreferExternalManifest=y
UseSbieWndStation=y
BlockPassword=n