Hello,
I'm propably overthinking it much, and the answer is really simple, but i just cant get it right.
I created two automation rules:
- If the issue is created by X, change "Request participants" field to user Name1
- If the issue type is "Onboarding", change "Request participants" field to user Name2
The rules both work fine, except when user X creates issue type "Onboarding".
In that case, both rules are executed, and they override each other.

Problem is, they don't always get executed in the same order (looks like it's random), so I cant just add "Copy from issue" to "Request participants" field, in the rule, that runs as second one.
Easiest fix would be, if the "Request participants" field added new user, instead of replacing existing.