What version of Go are you using (go version)?

$ go version
go version go1.21.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

$ go env
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/inso/.cache/go-build'
GOENV='/home/inso/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS='-buildvcs=false'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/inso/.gvm/pkgsets/go1.21/global/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/inso/.gvm/pkgsets/go1.21/global'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/inso/.gvm/gos/go1.21'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/inso/.gvm/gos/go1.21/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.4'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build484173072=/tmp/go-build -gno-record-gcc-switches'

What did you do?

Playground

I have created application in Go which collects data and saves the stream of data to the file using encoding/gob.
Later collected data is read and reports are produced. The amount of elements in the file can be large so I am reusing Data structure in the read loop calling Reset method to reset its state. Inside Data I have a slice of items Items []Item. To reset the slice and reuse the memory I do s.Items = s.Items[:0]. And when decoded data has default value of its type the decoder is not properly filling decoded slice.
What I assume is happening: if target slice has enough capacity the decoder expands slice to the target size (s.Items = s.Items[:1] in my case) and then it skips filling target fields if decoded item has default type value and decoded element got values from previous iteration.

What did you expect to see?

Ok: encData == decData

What did you see instead?

Error: encData != decData
encData: main.Data{Items:[]main.Item{main.Item{Id:1, V1:0, V2:32}}}
decData: main.Data{Items:[]main.Item{main.Item{Id:1, V1:11, V2:32}}}