egencache updates the Manifest BEFORE generating ChangeLogs. If it updates the ChangeLog, the Manifest is then WRONG; causing digest verification failures. I have implemented a workaround for infrastructure for the moment, but it needs proper fixing in the egencache code.
I've sent a patch for this earlier today.
Fixed in 2.2.24. https://gitweb.gentoo.org/proj/portage.git/commit/?id=fd69b6822c9929d011e2da863b8dad162b803088