Magisk Manager has silly bug there is unknown issue,SU access pop up never comes up after some time and even selecting grant default access the new apps never gets root access but the apps which generally has root access granted before the issue keeps working fine as usual.
Sometimes people reported flashing the magisk again works for them sometimes flashing stock rom works etc
In Magisk logs it displays : permission denied,su request rejected,write failed with 32 broken pipe etc
To fix it on those devices with issues, you can run Magisk Root Granter and grant root access (when root working fine with magisk) and when issue comes in future then select apps option select the app which asks for root and click save should fix the issue, i created this tool after i got fed up hopefully it helps others.