The 0 entrenchment ones were tanks, so that explains that. I wasn't aware of that rule. After looking at the info for individual subunits I think I understand what's going on, since f.ex. trucks also don't get entrenchment while infantry in the same unit does. I only looked at the entrenchment value for the whole unit which is probabaly an average of the individual ones. So for an infantry unit it would show f.ex. 81, although the infantry subunits were at 100.
However, it shouldn't be possible to play entrenchment cards on units where it will have no effect, such as tanks or units which are at the max entrenchent value already. The message also says entrenchment has increased, although it clearly hasn't and can't. Not a big deal, since I can check more thorougly myself in the future, but it is an issue from a user friendlyness perspective.
Also, the russians did entrench tanks by digging trenches for them so only the turret was visible at Kursk f.ex. so I think it should be possible to do in some terrains.