There are actually two locations along Roppongi Dori where this is the case. One by Metro Hat and one leading up to Zara. Both were exactly as you described. Up escalators run when it is wet and down ones are switched off with barriers placed to block access. I nearly fell more than once also. Fortunately, the strips are there now though as you suggest. It is through usage we learned. Kind of a "slip up" at the outset though.
Thanks for that report, it's good to know the building management is aware of the problem.
I've always felt guilty for not having been able to report it at the time.