There were updates available for the System 6 Appleshare client in the System 7 era (e.g., for accessing Appleshare 4 servers), so you might see if you do or don't have the most up to date client on the Pluses.

My experience after some years of trying to network old and new is that the most interesting and the most robust of all the bridges is the Apple Internet Router, which is rock solid and runs on an 030-040 machine. Gives life to such a machine, too.

Having said that, I am currently using one of those little white Asante boxes, as it needs to run 24/7/365. But even here, the order in which things get started matters with the thing, and I am not running netatalk.