I've run into an issue that when a networksync (manually or automatically triggered) is performed the bungee doesn't recive the ping after a few hours of running.
I'm using LP 3.2.7 on my ForgeSponge and Bungee servers. Both are up to date.

Interesting. The direction Bungee -> Sponge works, but Sponge -> Bungee doesn't!

So it works initially, but then breaks after a while?

I need more info really. Remember the networksync won't work unless there are players connected to that server.

Yes initially it works and then it breaks. I know that at least one player needs to be connected.

I'll happily provide any infomation you want. I just don't know what you could need.
But as I said, I tried it recently. A networksync (with a player connected!) on the SF server didn't get propagated to the Bungee, but a networksync on the bungee worked without issues.

It will just try every 10 seconds to send the message.

https://github.com/lucko/LuckPerms/blob/master/sponge/src/main/java/me/lucko/luckperms/sponge/messaging/BungeeMessagingService.java#L69

I mean the Sponge server tries to send a message. It even says so in the logs. But the Bungee never recieves it.

Are all of your players connecting through the same proxy?

Yes they are.

I don't know why then.

I could give you access if it would help with debugging.

Not really, your best bet is to try to find an error in your proxy logs.

Any luck finding an error?