Was commuting this morning and at Bethnal Green overground station there were a bunch of ticket controllers with their portable thingies. Interestingly Mondo card invoked some sort of error on each of their devices. One of them asked me if it is some sort of foreign bank card. I said “Yeah something like that”. They did let me go through the barrier, which worked fine 
Weirdly I had a similar experience about two weeks ago. I waited for the green light to switch off (mainly because some dude butted in), and at a second tap it worked.
Again this happened on the way out.