I've been to MAN Airport, several times in the last 12 months (T1 & T3) and in years gone by (mainly T1).
Never had any issues. I have a sensor (Libre 2). We can use the 'walk-in-scanner' with these attached to your arm, without it affecting the scanner (was informed by Free Style its fine) and since the scanner worked fine afterward, so I can confirm scanners are no issues.
What is an issue, is having a 'spare' sensor in my carry-on. These cannot go via the x-ray machine!!
This machines work differently to the walk-in-scanners.
First time, I wasn't aware, and when I changed sensor a few days later, yes it worked, but got (alot) more unable to scan readings.
When I checked with FreeStyle, they said simply to inform the security person at the airport.
Few weeks later, I was again at MAN Airport. I was ready for telling security not to place the spare sensor in the xray machine. But before I did, one of them came over to me, and said, "That cant go in the x-ray machine, I'll take it over there, do a swop and give it you back on the other side".
The following time, I did have to say something to the security personal, but they simply agreed, and didn't x-ray it and took it for a swop.
So I for one, have never had an issue with the MAN Airport (touch wood). I do however have one of those Lanyards, and go via the assisted area, not sure if they are simply better trained compared to the 'standard' security area.
As for your experience, as the others have said COMPLAIN.