The Minimum Grouping Digits for Spanish (Spain) is set to 2:
As a native Spanish speaker I've never seen such behaviour and for me the correct setting would clearly be 1.
I've seen there are other votes for using "1" so I'd like to add mine there.
Given that "es" is used for latin american variants, it could be that the right setting would be 2 for those countries, although in past releases, I've seen that es_419 override that value with "1", so it seems there may be controversy there too.
PS: We detected this issue by using Google Chrome, which uses ICU, and was reported to us by our customers that are using our web-based application with this browser. They switched to Firefox