iterate over non-empty table got empty

The original issue was reported in google group.

https://groups.google.com/g/openresty-en/c/4DXfmYSU7Jo/m/bUajTOB0AQAJ?utm_medium=email&utm_source=footer

i am testing my proxy server by new version of openresty (1.21.4), and i have a big problem
in load test
my nginx config just :worker 1;
and i have some code like this:

rewrite_by_lua_block {
    local t = {["1.2.3.4"] = { ["weight"] = 1}, ["4.5.6.7"] = { ["weight"] = 2}}
    ngx.ctx.t = t
}

balancer_by_lua_block{
    localt = ngx.ctx.t
    for key, value in pairs(t) do print(key) end -- output is empty
    for key, value in pairs(t) do print(key) end -- have output
}

Start with the linked Google Group report and reproduce the issue using the shown rewrite_by_lua_block and balancer_by_lua_block configuration. Compare both pairs iterations over ngx.ctx.t; done means the first iteration no longer produces an empty result or the behavior is clearly documented as expected.