OpenAPI.jl behind Oxygen.jl sometimes doesn't work

I’m new to web dev.
I’m trying to make a relay server which receives requests from users as a server implemented with Oxygen.jl and sends requests to another server as a client which uses auto-generated client code from an OpenAPI schema. When I use the auto-generated client directly from REPL, it works perfectly, but when I use it over Oxygen.jl server, it sometimes fails with the error “select/poll returned error” from curl inside Downloads.jl, especially when the request is close to the previous one less than about 30 sec.
So I switched a client from an auto-generated one to another auto-generated one which uses HTTP.jl to perform HTTP requests and it worked perfectly.
Why Downloads.jl one failed?

The actual code is here (WIP): MisskeyScheduledPost.jl/src/MisskeyAPIClient/Notes.jl at 5a5b405467d71f73be14737053cfaca970bc2a25 · qwjyh/MisskeyScheduledPost.jl · GitHub