I think It is probably because of glycerol, which has a boiling point of 290C and a melting point of 17.8C. I saw a similar thing with PEG before. If you want dried nanoparticles, you should purify to remove glycerol before drying your nanoparticles.

You had powder. Viscous glycerin was added and a gel was obtained. There was a structuring of the disperse system. Such systems have complex rheological behavior depending on the nature and composition.

Everytime there is an excess of surfactants it tends to form micelles, and in some cases, the micelles are the preferable "attach" point of the surfactants and it will "steel" the surfactants from the particles generating more micelles, in your case, I'd need to check with EM or other techniques, but I believe the glycerol formed micelles that might have traped some of the solvent inside, forming the gel.