it's not, actually. All info displayed move up and down the screen, exactly to avoid image retention/burn in. This never happened on the 830 i owned before, it's a new problem on this 830 they sent me.
Agreed--I've had two 830s (I sent one back because of an autorestart bug--an
actual problem), and neither had any sort of glance burn in when charging overnight. Mind you, I've got mine set to no background image, interval, and always-on while charging both times.
This seems more a criticism of the 930 than praise for the 830--the 930 is and was a substantially more expensive phone, which lacked expandable memory and glance, to great features that seem strangely absent (here I suspect Nokia and MS over them dropped the ball). It's the same sort of thinking that would get rid of wireless charging (as was the case with the 925, where you have to buy a special case, defeating the purpose). The phone
does work without them, and some people wouldn't miss the features, but you could find just as many people who wouldn't miss the 930's better performance.
The 830 isn't perfect, but prior to the 640's release (which
still isn't technically out in the US), it was pretty much the only handset that combined those three great features--expandable memory (which
every phone needs, we're not paying Apple $100 for memory that costs $15 more in MicroSD), wireless charging and glance--with a very good camera (crummy front facing camera though). And the 640 doesn't have wireless charging yet, as far as I can tell.
Nor is the Lumia 830 the first handset to be released that has weird, unexpected bugs (or even deliberate unworking features). An unlocked Lumia 925 can lose internet sharing when updating to 8.1. Seemingly
all the WP8 handsets that you can update to 8.1 have a music playback "cracking" issue. Compared to that, the 830 is a godsend.
Hardly high praise for Nokia and Microsoft, I'd say.