I haven't been involved in a long time but I saw this thread by chance and hopped on just because I worked on those components a few years ago.

I'd reccommend putting together a set of regular

fluent-bit -> fluent-bit

and

fluent-bit -> otel-collector

over

gRPC

configuration files to make it easier to reproduce and isolate the issue without extra cruft

You could also manually enable

http2

regardless of if you go through a "tradtional" regular HTTP/1.1 encapsulated

gRPC

packet in order to force fluent-bit to use the new HTTP client which will also try to use HTTP/2 if supported (although if you are going through a plaintext connection you'll have to set it to

force

in order to ensure that it doesn't fall back to HTTP/1.x)

That way you can get a bit closer to an answer (ie. determining if the problem is the new gen HTTP client dealing with the payload sizes, the gRPC implementation (at any stage) or whatever)

If you try to diagnose it on your own but find issues feel free to message me here so I get an alert and reply on a timely fashion since I don't keep tabs on the github repo