Please try to use a hash sign instead of a colon. For IPv6 support (where the IPs themselves contain colons) this is often used:
127.0.0.1#5335
EDIT: I’m shocked, it seems to be simply not supported by /etc/resolv.conf glibc. On other OSes (non-Linux) 127.0.0.1.5335 is a valid syntax to define a custom port but on Linux 53 is used forcefully. Interesting and disappointing, I didn’t know that as I always use an upstream DNS on servers.