Reposting here from X for visibility to others.

Core and Knots necessarily have the same service ID in order to satisfy dependent services like lightning.

Knots is on the community registry, Core is on the primary registry.

Your updates tab shows both registries, so you must have just clicked the Knots one.

It's an unfortunate and temporary UX shortcoming of 0.3.5.1 and will be fixed in 0.3.6 when we expand the versioning system to support alternative implementations of the same services, like Knots and Core.

To switch back to Core, do the following:

1. In the System tab, click the "manage" header 5+ times to enable developer mode.

2. Go to the Start9 Registry in the Marketplace. Click Bitcoin Core. And click "Reinstall".

3. Optionally disable developer mode by clicking the manage heading another 5 times.