EntryNodes is incompatible with IPv6-only bootstrap
When UseMicrodescriptors is 1, tor clients which bootstrap over IPv6 have to get descriptors from the fallback directories, because there are no IPv6 addresses in the microdescriptor consensus.
When EntryNodes is set, it excludes all the fallback directories, meaning that IPv6 clients can't bootstrap.
This can be tested with:
src/or/tor DataDirectory /tmp/tor.$$ SOCKSPort 0 EntryNodes x ClientUseIPv4 0