I’m having exact same issue.
All my Sangoma phones are 1 hour off. DST is setup correctly in EPM, and time is otherwise correct in the PBX itself. Logging into web portal of the phones and setting DST to “AUTO” also immediately fixes the problem like you stated.
I may make a custom basefile edit to force it to auto, hoping there is an easier global fix rather than editing each and every system and config file flavor, but that might be the way forward here.
We’re on FBPX 14.0.4.1 and Asterisk 13.22.0. EPM 14.0.2.59. Sangoma Phone Firmware 2.0.4.55 (S500 and S700) and 3.0.4.55 (S705).
I opened a ticket with support Ticket 844743