I don’t even know what a Purpur server is. The config is Instance → Configuration → Server Settings → Server Type: Bedrock (Official Pocket/Win10 Server).
AMP is running in an Ubuntu VM that has not changed. The instances all have the Docker option enabled.
Update:
I switched to the Minecraft Pocket/Windows 10 Edition (Experimental, then switched back to Bedrock (Official Pocket/Win10 Server), clicked the Download / Update button, and now it’s up and running again, and has updated to the newest release. The client can connect again.
Looks like some kind of bug made the config think it was Purpur? Even though it said Minecraft in the config interface all along?
I will try this same thing with the other Bedrock instances to see if it unsticks them, too.