I asked Atlassian support for help and they mentioned rate limiting for certian customers. I haven’t resolved it, but we can try to resolve it together. Please see Rate limiting Response handling pseudo code - #5 by dmorrow @veloti
BTW, how do you get that observation or conclusion that all the 6 REST APIs have this issue? May I know if your Jira instance is migrated from on-premise Jira (e.g. Server or Data Center)?