Interesting, I've only had a chance to use the latest version and currently went back to stock OpenWrt - so if you get a chance to test with more browsers be sure to report back.
Some similar reports go back to early in the summer - I can see the first time uci set uhttpd.main.http_keepalive=0 was mentioned in the thread was August 6th 2019 here: Davidc502- wrt1200ac wrt1900acx wrt3200acm wrt32x builds which also mentions a thread here Proposal (and solution!) for High Load fix on OpenWrt (LuCI) that goes back to Jan 12th 2019