DW 2021 doesn't remember Split > Code-Design state of documents.
After reopening documents, only code is shown. I have to manually re-select Split > Code-Design for each doc again.
This didn't happen in pre-2021 versions.
But it didn't make any difference.
I went back to DW 2020 - and there's no problem there.
So looks like it's just an obligatory new bug in a new DW version.
Hope Dreamweaver team monitoers this forum. I'll submit it to the bug forum, too.
Edit: Additional info:
Actually, as I was about to submit the bug, my own older bug submission regarding the same bug in both DW 2015 and 2018 came up. Which was eventually solved by resetting Preferences by holding Cmd-Opt-Shift when launching DW:
It worked this time in DW 2021 too!