Duration of defensive pacts in late game

My English civilisation is close friends with the Dutch and we've had a succession of defensive pacts to hold off the Carthaginians, Japanese, Americans, and Portuguese. I have G&K and BNW installed.

However, as the game progresses into the closing stages (1965 / turn 385), I've received notifications that our defensive pact has expired after only a few turns (varying from one turn to eight) instead of the correct thirty turns.

At first I assumed it was a glitch and that I was receiving the notification in error, but I checked and the option to sign a new defensive pact was there amongst the trade options in the diplomacy dialogue and the pact was not listed in the Your Relationships window.

I am aware of the glitches in the diplomacy system in Civ V, but is there some way I can resolve this issue of the defensive pacts ending so quickly? I have tried to avoid a bloodthirsty reputation, so if my ally is attacked I'd rather not be seen as being belligerent by having to manually declare ware. I do believe (although I am not 100% certain) that the existence of the defensive pact reduces the likelihood of another civilisation actually declaring war against one of the parties.