Since #3625 buffer-local options are reset to their default values when saving a file.

As a example, put the following into your init.lua:

local micro = import("micro")
local config = import("micro/config")

function preinit()
    config.RegisterCommonOption("plugin", "option", "old")
end

function onBufferOpen(buf)
    buf.Settings["plugin.option"] = "new"
    return true
end

function showoption(bp)
    micro.TermMessage(bp.Buf.Settings["plugin.option"])
end

and bind showoption to some key. Open a file and press that key. This shows "new". After (modifying and) saving the file, one instead gets "old".

The commit 698511c has this issue. The previous commit c61670e is fine.

CC @Neko-Box-Coder