there is another thread Access to sysupgrade.openwrt.org is blocked which gave me the idea to verify routes. something is fishy either with my ISP, or peer intermediate ISPs or openwrt.org's ISP or not sure, but using wan6 route has problems for sure. when i disabled wan6 it starts working.
so, no clue if openwrt server infra, ISP either end and ipv6 or whatnot caused the inconsistent downloading but it works now (via ipv4 wan).
thanks for the replies.