I have to say it was sheer luck, and it was a surprise even for me
I checked and you're right, the "l'" is "trapped" by the rules of the dictionary converter. I thought the reason was that the article l' is explicitly counted on the dictionary source, a really good one indeed. the funny thing is, the article d' it's there as well, but it's not trapped in "d'eau", for example.
watching closely at the french morphems.txt parsed by the converter, I noticed the verb "faire" was wrongly written, so I made a correction.
Compiled again the dictionary, and the article "l'" in "l'intérieur" it's not isolated anymore!
At the very end, I have no idea why it's happened. Maybe there's something going on when the converter create the index?
Anyway, I had to compile a Merriam-Webster collegiate, I'll upload as soon as possible.
the sources I used for this one and others are here
this is the fr.zip that contains the french rules parsed by converter.exe when compiling french-<anything> dictionaries, I've found it here in forums then changed a bit. Corrections and improvements are welcomed!