Not exactly. The TF mission does not hold the # of turns a TF has been at sea. The # of turns at sea is actually held in a separate location, and that number does get reset when a TF returns to its home port. The TF mission number holds the mission type, and in addition, it holds another value that is a multiple of ten and gets added to the mission number when a TF either completes its mission, or cancels it due to excessive combat with the enemy. For example, when a cargo TF completes its mission the mission number typically changes from 4 to 54, and when a transport TF completes its mission, the mission number typically changes from 5 to 55. I have seen air combat and surface combat TFs have 60, 100, or even 110 added to their mission number when the TF decides to return to port. These multiples are a normal part of the TF mission process that seem to indicate that a TF is now heading home and will be less aggressive on its return.
Now, I'm not positive on this, but I believe that if you don't periodically disband and then reform a TF, that is, if you keep the same TF together, and you constantly send it on new missions, then it appears to me that the TF will gradually get less aggressive, and eventually may even not carry out a mission. I have seen this occur on cargo TFs and bombardment TFs, where eventually they just will not go to the same base they have been going back and forth to for several turns. But if I disband and then reform the TF, all of its old aggressiveness returns. I have also had some success if I just reset the TF destination and home base, as this resets the TF mission number, and seems to keep the TF active and aggressive, at least for awhile.
Wow, that was pretty long-winded. I hope I've conveyed what I'm thinking, and I hope I'm not imagining this. But anyway, my normal practice is to always reset TF destination and home values (by reset I mean just setting the destination and home again) from turn-to-turn if I don't want to disband and reform a TF. Eventually (say once a month); however, I do disband TFs and reform them to ensure proper action.