[concernedcitizen76]

Perhaps.

In any case, for most of my life I can’t remember seeing contrails from jet airccraft that persisted for many hours and spread out to block the sun on a clear day. It’s only since the mid 1990’s that I began to notice this phenomenon. I have a great view from my deck, I watch the sky sometimes. I’ll see commercial jets flying leaving white contrails in their wake that disperse and disappear within a short time. On rare occasions, I spot, at a higher altitude above these commercial jets, two or three large white unmarked jet planes (no markings discernible through binoculars) criss-crossing the sky and sprewing out contrails that do no dissipate but spread out, as I say. It’s very odd.

[Dust in the Wind]

I understand and have seen same. But I am thinking that with time our suns out put is changing and with that naturally will be atmospheric changes. These could change the way these ice trails act. I don't know.
I have seen fighter jets in mock combat over the plains I live in and at high altitude those are the ‘tracks’ they leave in the sky.

[WhiskeyX]

“I’ll see commercial jets flying leaving white contrails in their wake that disperse and disappear within a short time. On rare occasions, I spot, at a higher altitude above these commercial jets, two or three large white unmarked jet planes (no markings discernible through binoculars) criss-crossing the sky and sprewing out contrails that do no dissipate but spread out, as I say. It’s very odd.”

There is nothing odd about it at all. The contrails become visible when they condense into ice crystals and/or water droplets. They disappear again just as soon as they sublimate and/or evaporate back into water vapor. How long the water is condensed into ice crystals and/or water vapor depends upon a number of factors including air pressure, air temperature, humidity, and insolation to name a few. The aircraft wake and turbulence in some circumstances creates temporary increases and decreases in atmospheric pressure along the aircraft’s wake sufficient to make the water visible and then dissipate again. In other circumstances, the atmospheric conditions are sufficient to make the condensation of the contrails persist for a very long time. In other words, it is an example of basic atmospheric physics.

[RansomOttawa]

In any case, for most of my life I can’t remember seeing contrails from jet airccraft that persisted for many hours and spread out to block the sun on a clear day. It’s only since the mid 1990’s that I began to notice this phenomenon.

That doesn't sound unusual at all. Wouldn't the mid-1990s correspond to roughly the time that high-bypass turbofan engines became widespread on commercial jets? Shouldn't be at all surprising that a different kind of jet engine produces a different kind of contrail.