It appears that some logs like reboot messages only appear on the in-memory logs and they don't get forwarded to remote log server,
is there any configuration i'm missing or is this by design?

You need to configure it to suit your needs, default configuration logs to memory only.

See /System logging, you need to:

1.- Create a remote Action setup for your syslog collector
2.- Set up logging Rules for the topics you want, setting/changing its action to the remote one you set on step #1.

Is there a way to send logs from Mikrotik to a remote syslog server over internet using TLS? Thank you