How to reproduce.
Create user parameter: UserParameter=test,echo OK
Request user parameter few times in parallel: zabbix_get -s host -k test

Result in our environment:
process1:
process2:OK
process3:
process4:
process5:OK
process6:OK
process7:
process8:OK
process9:

There is no such problem with built-in items (like agent.ping and so on).
There is no such problem in linux.