Another temporary workaround for margin problem for those who don't want to logout/lose all data:
Use KCC to convert to PDF and upload it via
https://www.amazon.com/sendtokindle
Use PNG and a 200 MB chunk size as needed.
This also enables you to write on the Kindle Scribe series.
You lose features like Right to Left unfortunately.
If you want to complain to amazon, you could say:
Fixed layout epub converted to azw3 via Send to Kindle has huge margins ever since 5.19.2 unless I logout.
KCC just creates fixed layout epubs, the margin problem is also on EPUBs directly from Humble Bundle uploaded straight to Send to Kindle, no KCC involved.