Yes, strategic bombing runs will damage land forts. The key here, is that you cannot repair anything in a province that is under attack, so if German AI maintains regular attacks on the Maginot while also strat bombing, eventually they'll knock them down. Even the repair national focus won't help, since they're under attack.
What's likely the culprit of your poor success in strat bombing is a combination of not attacking- and therefore not preventing repair- as well as poor RNG. Currently there is no way to target individual areas of an airzone, choose particular targets, or even to narrow down the list of potential targets for strat bombing. You may just be getting unlucky and your strat bombers damage infrastructure in Paris, or hit a civilian factory in Normandy. There isn't really good feedback on the results of your bombing runs other than "yep, stuff is indeed being bombed" I don't have information to back this up, but it seems the AI has some way of focusing fire on particular targets it wants to hit. I remember failing a Democratic Denmark challenge because my line of level 7 forts got bombed to the stone age, while my military factories and infrastructure never seemed to get hit.
IIRC, the only other way for improvements to be damaged is through partisan activity... which I'm fairly sure the French don't have to worry about in their core territory.
The big takeaway is that in order to maintain a wall of forts, you're going to want air control. Either standard fighters with upgraded weaponry or heavy fighters can knock back strat bombers, and static AAA in your states make a big difference as well. This may be just a theory, but I think static AAA reduce the odds of that particular state receiving strat damage? Might be a coincidence, but I found in a France game my fort lines didn't seem to get bombed with level 5 AAA in Alsace. That's just a guess.
Hope that helps!