This is a very very odd bug that I first time saw this, apparently, if you have those 3 mods together
Atlas-Lib-1.18.2-1.1.8.jar
restrictedportals-1.18.2-1.2.0.jar
The-Hordes-1.18.2-1.2.0a.jar

The infection doesn't work for the dedicated server at all, the player won't be infected despite putting 100% or higher, removing the restricted portal mod will fix the issue.
I am not sure if this is on hordes issue or on RP's side.

Note: Single Player works, only client joining dedicated server won't work at all.

Link: https://www.curseforge.com/minecraft/mc-mods/restricted-portals/

If the issue is on Restricted Portals mod side, please do let me know, I'll head over there and create ticket.

Closed as found an alternative.