The reason some red LEDs and the Thomas sodium safelights fog is because they also emit small amounts of non-safe wavelengths.
Look at the emission spectrum of a low pressure sodium tube. See all of those small blue and green spikes beyond the dominent sodium doublet? There's no way to visually detect those in the flood of monochromatic orange light, but they are there. Remove them (using a Roscoe #19 Fire filter) and the fogging disappears.
Same for the red LEDs. Remove them (using a Rubylith filter) and the fogging disappears.
Try this. Hold up the business side of an ordinary CD disc to your red LEDs (or to a Thomas tube) and look at the discreet wavelengths that are reflected. I'll bet if you look carefully you'll see a few narrow green and/or blue stripes mixed in with the red or orange. I know I did.
My red LEDs showed small telltale green bands and my Ilford MGIV VC paper was faintly fogging. The Rubylith cured that out to 60 minutes tested. My Kentmere Bromide graded was severely fogging under the Thomas. The Roscoe cured that out to 30 minutes tested.