I’ve never heard of this but if I were you I would make a couple of screenshots before and after collecting.
Before collecting, on the first tab titled “YESTERDAY”, it will tell you your position and below that the number of MEBC coins you earned. For 70 coins you would finish between 1000th and 1999th position. Take a screenshot of that first.
Then before collecting go to the “TOP PLAYERS” tab, it will tell you your Total Mighty Eagle points in this season on the left, take a note of the number and screenshot it.
Go back to the “YESTERDAY” tab and click “COLLECT”. If the number shown isn’t the number it said you would be collecting then screenshot that too.
Finally go back to the “TOP PLAYERS” tab, it should be the number collected on to your previous total on the left. If not screenshot that.
Finally send a report to Rovio. I’m sure they will compensate you for the missed coins as you have highlighted a bug.
To answer your question, I’ve not heard of this particular issue before.