Thanks for a bug report. I see it, but it seems more like the value is not read, even it is used, thus the problem might be more UI, than background. For me the use of the "hidden" socks proxy is proper, my IMAP accounts refuse to connect if I entered into the settings some garbage, like "socks.no.where:3210"

Created commit 34e95c4 in evo master (3.13.7+) [1]
Created commit 47a537a in evo evolution-3-12 (3.12.7+)

The first chunk, at e-util/e-proxy-editor.c, makes perfect sense, there was forgotten to "load" the values to the SOCKS widgets, but the rest, at e-util/e-proxy-preferences.c, feels rather hackish. The problem is that if a user closes evolution within two seconds, then the changes are not saved, which the rest of the patch tries to avoid and it is supposed to save changes immediately when the Preferences window is hidden. Even that can be sometimes too late, but it's still much harder to reproduce the "changes not saved" issue.

[1] https://git.gnome.org/browse/evolution/commit/?id=34e95c4