tk provides incorrect data when setting _NET_WM_ICON which causes odd problems including xprop reporting "out of memory" and fluxbox displaying empty/corrupt icons. According to upstream, this will be fixed in 8.5.9, and if you want to backport the fix, it's apparently CVS version 1.58.2.4 for file unix/tkUnixWm.c. I may try to extract the patch and attach it to this bug too, time permitting.
Created attachment 242279 [details, diff] tk-8.5.8-64bit_wm_icon.patch Good news, simply extracting the patch from upstream CVS seems to apply cleanly and fixes the issue! Patch attached. By the way, I have this ready to go; I can commit tk-8.6.8-r2 with this patch if you would like me to do so. Just say the word.
Fixed in 8.5.9