As explained here: https://github.com/odoo/odoo/commit/770aec399c4e6e29cfc5854318cbda19a605315d#diff-3d7991d8bc534e0372f0bd9b4f9165a7R424, we have to post-process the generated css to add some vendor prefixes. Doing that there was however only adding those in non-debug=assets files.