How does this happen?
For melted filament to leak around the heat break threads, it has to first get through the metal-to-metal machined surface joint where the heat break contacts the nozzle. Given this keeps happening, the problem must be a systematic error of some kind.
Thermal Cycling
Based on your description of your installation procedure (heat to 285 °C, tighten, cool, reheat to 285 °C, retighten, cool, print at 240 °C) I see a potential issue: titanium is subject to brittle fracture. Your titanium hot end and nozzle, together, are being overstressed with the combination of heating well beyond print temperature, overtightening, and cooling (the aluminum heat block has a much high expansion coefficient than titanium).
Suggestion
What I'd recommend is to change to a brass nozzle or, if you need to print with abrasive filament, hardened steel, and a 304 or 316 stainless heat break. The steel or brass nozzles will cost less than titanium and do at least as good a job (brass is best for thermal transfer to the filament, though it has limited wear life), while stainless steel for the heat break will have similar or lower thermal conductivity and is less brittle. As a bonus, with your tightening routine, the brass nozzle material will deform when it is compressed against the stainless heat break and act as a gasket, rather than fracturing after a few heat/cool cycles.
An Additional Concern
While answering comments, I was reminded of another possible issue: the same reason aluminum wiring is no longer used for in-wall home electrical installations. Aluminum tends to deform permanently after heat expansion under constraint, rather than recovering its original shape (as copper and brass will do), and expands several times as much per degree as titanium does. Even if the heat block is an alloy rather than conductor grade pure aluminum, this will occur (though most alloys are significantly harder/stronger than pure metal) with the heat, tighten, cool process. Also, 280 °C may be hot enough for aluminum to undergo heat creep (a slow, permanent deformation under stress normally well below the yield point). The rule I recall from material science courses in college, almost forty years ago, is that creep becomes an issue when the absolute temperature is more than about half the melting point, for most metals, and pure aluminum melts at about 933 °K while you're printing at 553 °K (by comparison, 70/30 brass, as used for firearm cartridges, has a solidus -- the lower melting point of a two-component alloy -- of about 1183 °K.
This permanent deformation of the aluminum heat block at the threads may result in the threads becoming loose, thus allowing extrusion pressure to force molten filament between the heat block and heat break. That layer of filament material then acts as an insulator, increasing the temperature differential between the heat block and heat break -- and with the heat break now running cooler, the gap opens further.
A Suggested Solution
One solution to this would be to switch to a brass or copper heat block. Not only do these metals have a higher melting point than most aluminum alloys, making them less prone to creep at printing temperatures, but they have much more tendency to return to original dimension after a heat/cool cycle when constrained (which is why copper house wires don't become loose over time the way aluminum ones do). The change in mass and conductivity might make it desirable to recalibrate the temperature control PID, if present, but this is a one-time operation, compared to having to disassemble and clean the heat block and heat break periodically due to leaks.
