UNDERSCORE officially not accepted in the SSID (WiFi) for the Fenix 6 series (despite it works with the Fenix 5x)

Which AP (make/model) do you use?

the watch has always been able to conect to the wifi even with the  dot and underscore in the SSID, I can tell you this as far as the wifi setup both from the watch itself or from the app (mobile or pc) was working with no error code but most of all cause I could check on the router that the watch got an IP assigned.

reguarding the sync....using the verysame same wifi, just renaming the SSID (different name wuthouth the special chars, and of course repeating the wifi setup procedure) results in sync working. Reventing the SSID name results in sync not working again. Everything esle in the router and wifi configuration stays the same

Linksys WDR3200 with openwrt firmware

Huawei P30pro hotspot mode

Huawei P30pro wifi bridge mode (works as an AP to the local WIFI)

netgear R6400 with asusWRT firmware

I only remember having issues with other Huawei phones in the past and never used the Netgear, sorry.

But I do have the Linksys with DD-WRT v3.0-r44048 on it as my main router and I know that one has worked with our Garmin watches here. But I don't use WiFi on that one anymore, since my ZigBee-GW sits right next to it.

I use three old and cheap APs from TP-Link (tl-wa850re), with those even this works:

what makes things more awkward is that my old Fenix 5x could sync via the same WfiFi (SSID with _ and .)

but the 6x can not

I understand, but from a network engineers point of view, one should avoid using underscore, period and other special characters across platforms. And not only when naming SSID's, it's also true for domains and workgroups, hostnames, usernames and passwords.

This may also be why you got that slightly odd answer from Garmin, they simply played it safe.

Yet mine, and others, can sync repeatedly using "_" and "."

What is the SSID you're using? Happy to try it out.

The mysterious SSID, will we ever know?