Your expectation is correct, but perhaps your knowledge is lacking. Around the 13th century the earth was the hottest it has been until recently. It's called the Medieval Warm Age. Sea levels were higher and started retreating around 1300.

Edit: To add, this cites some sources. It's not a massive change in sea level obviously, something between 1 and 2 m.

[–] Ephera@lemmy.ml 5 points 8 months ago

Hmm, interesting. The graph I posted would be a bit too linear, wouldn't it.

But it's actually not the entire Earth that was affected by the MWP, but rather just the North Atlantic region (the Wikipedia article specifically says so).
I guess, some of the Northern polar caps melted, so maybe it did affect global sea levels anyways. Or it might have been something like the land heating up, causing stronger winds from sea to land and that just pushed more water towards the Netherlands. Honestly, I'm just spitballing at this point...