Image capture rate changes mid-flight

We’ve just flown a mission with 80% front and side overlap. Mid way through the flight I noticed that the frequency of the images being taken (represented by the small FPV window on the app blinking black) had dropped down from its normal rate of one shot every two seconds to one every ten seconds. I was hoping that the app was just having a problem but on returning to the office and downloading the images it has persisted. In the first half of the mission I’ve got 30 shots per minute and in the second half I’ve got 6 shots per minute. Clearly this ruins the desired overlap so it will need to be flown again. Can anyone advise why the drone/app/mission exhibited this behavior please?

Already had the same issue a few weeks ago.
No solution but flying again … Grrrr !!