What is the military regulation on wearing the fleece jacket with civilians? I'm unable to find it. Is it authorized or not?

So the problem with AR 670-1. It doesn't cover cold weather gear as it pertains to the ECWCS. All of that falls under a TM, as it is a "cold weather system." That's right. A TM! complete with a PMCS guide. Anyway AR 670-1 as well as DA PAM 670-1 will say this.
"Uniform items authorized for wear with civilian clothing by males are restricted to the gold cuff links, studs, tie bar, mourning band, footwear, socks, gloves, undergarments, black bow-tie, wool scarf, all-weather coat, fleece caps, and physical training uniforms."
So one would naturally believe that the fleece jacket isn't included in this therefor isn't allowed. But the fleece jacket has zero mention in the entire regulation. Because AR 670-1 does not govern ECWCS Gen III.
DA PAM 670-1 does mention ECWCS. So because the same quote about what can be work is in the DA PAM. I would say No, not authorized.
Things to note. AR 670-1 also has this little blurp.
"Commanders will not prescribe seasonal wear dates for uniform items, but may prescribe uniform(s) based on safety reasons (for example, for extreme cold or hot weather based on temperature)."
Commanders love to ignore that one and say "_____ is only authorized between 'insert month' and 'insert month'

I mean, you could google it....

Look in AR 670-1 and DA PAM 670-1. You can find these publications on the Army Publishing Directorate website. CTRL+F and type in fleece.

If you're 100 miles from the flagpole, I'd guess there wouldn't be anyone around to tell you it's not allowed. Don't you own any NICE civvies?

You could always just buy a civilian fleece jacket and not have to worry about it.

AR 670-1
Paragraph 3-9(g) Uniform items authorized for wear with civilian clothing by males are restricted to the gold cuff links, studs, tie bar, mourning band, footwear, socks, gloves, undergarments, black bowtie, wool scarf, all-weather coat, fleece caps, and physical training uniforms.

The fleece jacket is not authorized with civilian clothing.

There should be one somewhere in 670-1. There is a reg regarding the lacing of boots. If you cannot find it, don’t wear it.
Personally, I never concerned myself with what can be worn with my civilian clothes because when I’m off duty, I don’t want to wear uniform parts.