GNU Radio | Packet Encoder/Decoder | Unwanted bytes added to the transmitted stream
Packet Encoder and Decoder are broken; they drop data.

That's why they are in the deprecated category (for years now!). We've removed them, because as a project, GNU Radio has not been able to fix them (and also, they were terrible from an architecture point of view).

So there's exactly one solution: don't use packet encoder / decoder.