If your heater is like mine, the lower line sits just a tad below the tank, so water tends to gather at the backside of the bypass valve. Discovered that this year after I blew out the lines, but didn't fill the lines with anti-freeze right away. Had a cold snap sooner than expected. I found the valves iced up, so I ran the furnace and brought in a second heater, then ran antifreeze through the faucets.
What I do now is when running the AF through, when finished I crack the lower bypass valve for a moment to get a little AF behind the valve to keep the water behind it from freezing too hard. In spring once the lines are flushed out, then open the lower bypass and flush the tanks once more before putting the plug back in.
Even blowing the lines, some water finds it's way back in at the low spots, so better safe than sorry.