I looked at my 2003 TC40.
The PTO handle is held onto the short shaft going into the PTO clutch control with a roll pin. Assuming yours is the same, and your roll pin is not missing or sheared off, there is no way for the handle to slip on that short shaft, or to adjust the handle's position with respect to end of the guide slot when it is engaged. I suppose a bent handle is a possibility but doesn't seem likely from normal use.
The tension on mine seems to come mostly from friction/rubbing of the handle in the guide slot, which narrows at the point the PTO clutch begins to engage. That is not going to be easy to change (tighter or looser) by the looks of it. Sort of hokey IMO but it's been working for 11 years. There is some internal drag and I can feel a detent when the PTO shift handle is almost all the way in the off (forward) position.
Mine also has an adjustable stop bolt for PTO safety switch. I've never adjusted that but there may be instructions in the manual. It doesn't look like it could possibly affect the PTO engaged position or the amount of drag.
At this point, I would make up a loop of rope or use a bungee to keep that handle from creeping out of full engagement if you can't see any reason why it slips out. If that is the only issue, then at least you will prevent damaging the PTO clutch. If there is another issue, it will show up eventually. Always engage and disengage the PTO at your lowest engine idle speed. Before disengaging, give the mower a chance to slow down so the PTO brake is not working any more than necessary to stop the rotating blades.
Let us know how it goes please.
