This is new behavior in 2.1.0 that wasn't present in earlier versions.
When you create a random draw part in an assessment, the question pool you draw from should be associated with that part and not available for further use in that assessment. (That's if I understand the notes in
Tested the following (separately) on all 3 nightly instances:
1. Created random draw part, followed by rd part using same pool
2. Created random draw part, followed by regular part with questions imported from same pool.
3. Imported questions from a pool to a part, then created a random draw part drawing from the same pool.
I was able to do all 3 of these on all instances, although after doing #1, previewing the assessment threw a nonfatal exception. (see attached).
#1 should definitely not be allowed. Possibly #2 and #3 should not be allowed either as they risk accidentally duplicating questions within an assessment. But at least they don't break anything.