The DB is the same. Sometimes the brew boiler drains and sometimes it needs the 1 cup button press to get it going. It's probably down to water and surface tension - just needs a bit of help to get it going. The drain pipes are narrow dia. They have to take high pressure so they use the same pipe and connectors that are used elsewhere.
The boilers hold ~900ml so 1l is a bit close. I used 1.1L. Sage's own engineers use Puly descaler.
The DB uses the same internals. Some people don't descale and say that is when the machine has problems. They may be using certain brands of bottled water to avoid descaling. It's the fact that they aren't descaling which probably causes the problems. Same could happen with tap water if not done often enough. Sludge can collect in the bottom of the boilers and descaling probably cleans the level probes in them. Problems like that can cause the valve error that can crop up during descale - tanks not draining when they have.

If stuck there is a video on youtube concerning descaling the very early DB when it didn't have the drain taps.
I've never had my machine stop fully reheating the water after a descale. I don't skip out of the procedure early and always allow the full 20min before dropping out.

But maybe I have been lucky. Other than here and recently I haven't seen any mention of this problem. Having a tap open causing it does make some sense as the machine can decide to put water in the boilers at any point in time even when powering up. However when steam is being drawn it doesn't. It probably does at some point if enough has been drawn off and that seems to be way way more than what people use in practice. It may decide to add water during a descale.
When people have steam problems Sage tell them to descale the wand. So when I descale I use the steam selector briefly a few times and just let a bit of wet stuff come out rather than steam, say 5 min apart.