I have 4 of the Z3’s, Kitchen, office and 2 bedrooms. They have worked perfectly with Ethernet for years. Then I updated the one in the office to the latest firmware 1.0 (build 56). Since that firmware update the Z3 has experienced the issue your having with it disappearing from the Network every few week to a month. And yes, power cycling the Z3 gets it reconnected to Roon for the same period of time, a few weeks to a month.
I decided to leave the other Z3’s on the 1.0 (build 55) version and they stay connected 24/7 without fail.
I keep hoping another firmware version will come available and fix the issue but It’s been several years now with nothing happening.
I did open a ticket with Roon, see below.