Sporadic cURL error 28: Resolving timed out after 5000 milliseconds

[nine-2-five]

Hi, I'm using WordPress FPM 8.0 with the official NGINX image as a reverse proxy. I'm getting cURL error 28: Resolving timed out after 5000 milliseconds, sporadically, on pages that use wp_remote_post to get data from an Express.js API in the same datacenter (but different domain). The proxy I believe can be excluded from this problem, because the same error was logged with Open LiteSpeed proxy also.
The wp_remote_post gets the data 99% of the time, but I'd still like to know why this error happens 10-20 times a day.
Also, I had the chance to call the API using Postman within seconds of error being logged, and it responded within 100ms on each call I made.