I posted a bug report for the speed restriction issue on descent a short while before the latest update. It was acknowledged and put on the list, so there's possible improvement for that issue in the future.

For the speed constraints, I've found the aircraft to follow on certain occasions and disregard on others, so it's difficult to pinpoint the manual references and describe how it functions for a bug report.
Same for SEL SPD - it sometimes locks to 240kts regardless of input and then for others it rapidly accelerates to 400+kts like in the screenshot; it's hard to summarise in a bug report.

Hopefully the 757 can be refined further in time. The descent phase is really letting it down now.  :-/