Opened a support case.
First reaction: HTTP::uri is not supported in the ASM_REQUEST_DONE event.
Wrong.. it is.
But there is a bug:
ID429063 - "ASMiRule: error during executing HTTP::method, HTTP::uri on ASM_REQUEST_DONE/VIOLATION EVENT for 100-Continue."
This behaviour ( of the TCL error ) is marked as "FUNCTION AS DESIGNED", for 100-continue requests only. This behaviour is due to a fix that was created to anther bug, ID392895 - "iRule: Incorrect data in ASM_REQUEST_VIOLATION, ASM_REQUEST_BLOCKING events in case of violations in both parts of 100-continue request".
100-requests are requests sent with header "Expect: 100-Continue".