QUESTION:
Doe the blocks FC5 (PLC_ SEND) and FC50 (PLC_LSEND) behave differently with respect to the DONE bit?

ANSWER:  
Yes, the block "PLC_SEND" (FC5 V1.2) keeps the DONE bit at TRUE just for one cycle. The block "PLC_LSEND" (FC50 V 3.0) keeps it at TRUE until a new order is started. This concerns only the S7 300 CPs up to CP343-1 (MLFB: 6GK7343-1EX10-0XE0). S7 400 CPs and more recent CPs of the S7 300 series are not affected by this behavior.

Note that for the Startup procedure the first call of the "PLC_LSEND" without order ("ACT" is TRUE) also keeps the DONE at 0.