The modern way of thinking about security of internal networks is to assume there is no such thing (marketing term: Zero Trust).

I'd consider it a concern if I was in your shoes, yes. Mostly for the reasons jflorez mentioned.

Major security breaches wherein an attacker gained has access to an internal, private network happen not infrequently. Target (the retail chain) leaked a ton of their customers' credit card to attackers over the course of (IIRC) months. The attackers couldn't have done it (at least not in the way they did) without first breaching Target's private corporate network.

Never underestimate the risk of an attack coming from the inside.

Also once you have an implementation with a certain kind of authentication other devs are likely to copy what you have successfully deployed and then your security assumptions will make it into public facing code without much consideration