I had the map fully generated and then when I switched the world the map became blank and no longer updated the tab label for it with the server motd. It's just completely white and has absolutely nothing on the page.

there is a screenshot of what it looks like

also just to clarify:
Server version: 1.15.2
I have tried rebooting the server
I have tried switching to a new world
I have tried doing a full render, it appears to be rendering with the chat messages, but the map site is still blank.

Guessing its a user error. Try renaming dynmap folder to something else.

I have the same issue,nearly exactly the same. The web page shows you, and I know the port is not used as I used a un-used port generator on shockbyte. (the provider I use). I get the same blank web page, as shown in the screenshot.

Here's the link :
http://heliosphere.mc-shock.com:3441/index.html

If anyone knows whats going on please tell us. Thanks

If you're on chrome try inspect element, and see if you're actually being served a dynmap page