[Q] "Android is upgrading" "starting apps" Every re-boot topic

I have a Z3 Compact and a Z2 Tablet. Both have been upgraded to 23.1.A.1.28. With the previous 5.0.2 build, everything seemed fine but with the minor upgrade last week, I now get the message: "Android is upgrading" "starting apps" whenever I re-boot. Not a big issue as the reboot continues after 10 seconds. As this happens on both devices, I am assuming it's something to do with an app that's on both of the machines but I don't know enough about this stuff to know how to identify where the issue lies. Would it be a case of disabling all apps and bringing them back one by one? I have hundreds of apps so want to avoid a factory reset if I can.

I am also thinking that it is not a common problem as I have trawled around and only see reference to this with rooters and flashers so again, assume it is something common to both my devices rather that a system issue.

Any pointers please?