Defensive Works Downgraded?

[MisterMark]

I'm playing a scenario in Salerno43' as the allies and I have established several defensive works around some key objectives I took much earlier in the game (I'm about halfway through a 275 turn scenario at the moment).  Most of the defensive works have been established 20-30 turns ago if I had to guess and are mainly 'TRENCH' with a few 'IMPROVISED' here and there.

I know that if you vacate a hex with defensive works and you or the enemy reoccupies it, it will be downgraded with 1/2 it's strength and noted as such with lower case letters, ie 'trench' or 'improvised'... and I've learned you can easily get it back up to full strength by having an appropriate unit 'dig-in'.

However, on my last turn I noticed several of my works were downgraded to 'trench' or 'improvised' and I am 99% certain I had never vacated those particular hexes.  I didn't see anything in the manual regarding this and I'm wondering if it's a bug or some game mechanic that I don't fully understand yet.

Anyone have any idea?

Thank you,

-Mark