I can confirm that I'm getting the same.
I also noticed that running gofmt instead of go fmt it seems to be working as you expected.

$ go fmt ./...
$ go fmt ./bar
bar/bar.go 
$ gofmt .
// +build integration

package bar

func Bar() {
}
package main

func main() {
}

I'm not the most qualified person to ask, but I think that the behavior might be intended; gofmt just traverses the directory tree and formats the files, while go fmt works as the go command (same as go build or go run), in that it respects the build tags, and ignores packages that have some build constraints.

On the other hand, I'm not sure whether the tags functionality is currently implemented for go fmt, so we might want to include it, in the same way that go build does.

go fmt works as the go command (same as go build or go run), in that it respects the build tags, and ignores packages that have some build constraints.

Tags apply per-file, not per-package.

I think that current behavior is not consistent.

First, go fmt ./bar formats bar/bar.go while go fmt ./... does not. ./... should include ./bar.

Second, if I put a file without tags into bar/ then go fmt ./... formats bar/bar.go, so it is not the case that go fmt just ignores files with tags. It ignores them in a very specific case. In normal circumstances go fmt ./... formats all the files. So it looks more like a bug, than an intended behavior.

Also people rely on go fmt ./... to format everything. I found this issue when I run go fmt ./... and found unformatted files in some directory.

For what it's worth, I format with gofmt and not go fmt, one of the reasons being that I want to format all Go files in a directory tree and not bother with packages. Though it does sound like there might be a bug here - see #22183 for a similar one that we fixed a while ago.

This does seem like a bug.

go fmt expands command line arguments into a list of packages the same way go list does. It then formats all files in each package.

When expanding patterns like ./..., the package loading infrastructure ignores directories were all files are excluded by build constraints, so that ignores bar in this example. Those packages will be returned with an error when they're named specifically (./bar). go fmt specifically ignores that error and formats all .go files including excluded files.