If calendar alerts don't sync properly between all my devices

This is probably a feature, but here's how I got rid of it after multiple tests.

Findings:

In the Alerts settings in the Calendar app (whether macOS or iOS), no matter what I choose, this Alert: 

- Will only apply to the device I'm currently on -> No synchronisation between my devices

- Will be added to the one chosen when the event was created (for example if this event was created on another device)

Resolution:

I was able to solve my problem by applying the following settings on each of my devices:

1. [macOS + iOS + iPadOS] Disable Alerts by default (choose "none" for all Events)
2. [macOS] Uncheck the "Use these default alerts only on this computer" box 

(even unchecking this box doesn't synchronise this setting with my other Macs anyway...)

Now when I create an event I manually choose an Alert (none, 5mn before, 1h before, etc).

-> This alert is now correctly synchronised between all my devices.