Here is what appears to be a bug with the "transmit message" triggered action behaviour in the mission editor. I have seen reports about similar bugs, but found this behaviour only for copy/pasted units.
Here are the steps I followed to reproduce this issue:
1. I created a unit (AH-64D -default name Rotary-1) as "Takeoff from Ramp" and uncontrolled, with an assigned frequency of 315 MHz, with an assign a "Transmit Message" Triggered Action named "TM1", with "Message 1" as subtitle and "BLIP1.ogg" as sound file.
2. I then copy/paste Rotary-1, which becomes Rotary-2, and inherits the same Triggered Action.
3. I then change the triggered action for Rotary-2 to name "TM2", subtitle "Message 2", and "BLIP2.ogg" as sound file.
4. I created a third unit normally (not using copy/paste) with a "Transmit Message" Triggered Action named "TM3", with "Message 3" as subtitle and "BLIP3.ogg" as sound file.
Expected Behaviour:
1. I would have expected the triggered action of these two units from different groups to be independent, and that any changes to one's triggered action values would not have any effect on the other's.
Observed Behaviour:
1. After changing Rotary-2's values, Rotary-1's sound file changes to "BLIP2.ogg", and If Rotary-1's sound file is changed to BLIP1.ogg, Rotray-2's then changes to match. None of the other values (name or subtitle) are affected during these manipulations.
2. However, if the mission file is saved, Rotary-1's subtitle reverts to Rotary-2's subtitle ("Message 2") while the names are unaffected.
Note:
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A. If the new unit is created using copy-paste before assigning the transmit message to the original unit, this behaviour is not present, i.e. the subtitle of new units is correctly maintained.
B. The Transmit Message Triggered Action values for a new unit created without the use of copy/paste work as expected and is unaffected by changes to other units' values.
C. If the "transmit message" is added as an advanced waypoint action, this behaviour is not present, i.e. the subtitle of new units is correctly maintained.
D. This has been verified with ships, ground units, helicopters, and fixed-wing aircraft, all with the same results.
E. Background auto-backup do not have any effect on the behaviour unless an auto-saved file is recalled/opened.
F. The last sound file to be selected by either of the copy/paste units is the only one saved in the mission (.miz) file. (BLIP1.ogg or BLIP2.ogg). The sound file associated with the non-copy/paste unit is saved normally in the mission file.
Transmit Message Test.miz