Forge version 31.1.18
Minecraft2Discord version 1.15.2-1.1.3

Every other advancement a player makes does not get sent to in-game chat. This mod sometimes blocks it from being sent as well as deaths and anything else related to a player. It catches the event yet blocks Minecraft's event being sent to chat.

Hi,
Thanks for reporting. Is it possible to have more information? What's the list of mods that is installed, did you have any error in the console?

Thank you Denis for responding. I currently do not see anything in the logs with any Minecraft2Discord errors. I removed Lithium as I saw it was not needed, the server runs smooth as silk without it also might have been the culprit. Here is the modlist
I haven't noticed it skipping things out of Minecraft chat since I removed Lithium.

Yes, the issue stemmed from Lithium. I removed it and no issues yet. Closing this, if I decide to run Lithium in the long run I will get a hold of the dev. Thanks again and sorry for this report.

If the issue is resolved, it means that the issue is coming from it... Can you confirm that the issue is resolved? If it is and if you want to play with lithium installed, let me know it and I will try to reach lithium devs in order to find the origin of the issue

There's nor problem for reporting... It's sometimes difficult to see from where is coming the problem.

Ok it is still doing it. No errors on server, nor client. Here is an example
Tail of server log:
[12Mar2020 16:39:05.296] [Server thread/INFO] [net.minecraft.server.dedicated.DedicatedServer/]: <shortybsd> yeah i am good [12Mar2020 16:39:15.217] [Server thread/INFO] [net.minecraft.server.dedicated.DedicatedServer/]: <shortybsd> dude this is freaking awesome !~ [12Mar2020 16:43:39.934] [Server thread/INFO] [morpheus/]: frankieleft is now sleeping. 1/2 - 2 miners (100%)

Screenshot of Discord

Screenshot of Minecraft Client

Screenshot of my advancement with nothing being sent to Minecraft chat, however you can see discord notification showing it from Discord

So your problem is just the advancement that is not sent in the server chat but relayed in Discord? I can't see anythings causing that in the code... If you remove only minecraft2discord is the advancement send in the server chat?
If you want you can join my discord so it's easier for us to chat : https://discord.gg/rzzd76c

That's what happens to invisible advancements. as per #31

Isn't exactly the same case while here the advancement is suppose to be displayed on both side but it's displayed only on discord.

You are sending a message to Discord for every advancement here: https://github.com/Denis3d/Minecraft2Discord/blob/3af7c4e9944e154fd1b933669e23c07ba222b958/src/main/java/ml/denis3d/minecraft2discord/events/ServerEvents.java#L97

Minecraft handles advancements for the in-game chat by itself. It filters out any advancements that are supposed to be hidden.
Like that you end up with the situation above.

Yeah but in this case minecraft is hiding advancement that is supposed to be show

I'm closing this as there is no answer from the issue owner for 1 week