WLP036 (Düsseldorf Alt) and cool temperatures

The OG value is not the problem. You have to understand that Altbier is fermented at lower temperatures. That's what the style requires. Deviations are not allowed. Altbier is a mixture of ale and lager and you absolutely need 16°C or less to get the appropriate flavour. It is one of the most difficult styles of all, as everything has to be balanced.

I wonder about this: normally an Altbier yeast like WLP036 Düsseldorf should also work in its low temperature range; 18-21°C is for Ale, but normally a yeast like W165 from Weihenstephan is intended for 16-18°C.

Maybe White Labs forgot the additional specification on it? I have sent a message to WLP but have not received an answer yet.

14C is 57F or so. That awfully cool for an ale yeast. Even 16C is only 60F. That's still below their recommended range. While it may be recommended to ferment at lower temperatures, 4C/8F lower than WL's recommended minimum will definitely slow, and possibly halt, fermentation.

You said that initial fermentation was "extremely vigorous" so this drop in fermentation activity may be due to finishing fermentation rather than due to temperature. 1.051 wort isn't particularly high gravity, so it's possible that with that large of a starter fermentation may be complete.

I suggest taking a gravity reading and comparing that to your expected FG. That's the only definitive way to tell what is truly going on.