Unable to obtain lock on KV

#### Unable to obtain lock on consul KV pair, getting error : " Existing key does not match lock use"

I'm trying to write a code which will create a new KV pair if it doesnt exist, if exists it will update the value.
Below code is used for the same:

#### Reproduction Steps

Steps to reproduce this issue, eg:

1. Running consul in dev mode on mac
2. Using the below code first i'm creating a new key pair in local consul

```sh
func acquireTask(taskId string) {
client, err := api.NewClient(api.DefaultConfig())
if err != nil {
panic(err)
}

opts := &api.LockOptions{
Key: "tasks/"+taskId,
Value: []byte("set by sender 1"),
LockTryOnce: true,
SessionOpts: &api.SessionEntry{
Behavior: "release",
TTL: "10s",
},
}

firstLock, err := client.LockOpts(opts)
if err != nil {
panic(err)
}

locked, err := firstLock.Lock(nil)
if err != nil {
panic(err)
}
fmt.Printf("=====> result for first lock is %v", locked)

defer firstLock.Unlock()
}
```

3. Now next go to consul ui and edit the key value pair that u have created and save it
4. Rerun the above code to get a lock, u will get the below error
```panic: Existing key does not match lock use```
5. lock.go line 207 is throwing the error

```sh
if pair != nil && pair.Flags != LockFlagValue {
return nil, ErrLockConflict
}```

consul logs show the below:

```sh 2021-03-30T13:05:38.053-0500 [DEBUG] agent.http: Request finished: method=PUT url=/v1/session/create from=127.0.0.1:59982 latency=326.547µs
2021-03-30T13:05:38.053-0500 [DEBUG] agent.http: Request finished: method=GET url=/v1/kv/tasks/unassigned_1249?wait=15000ms from=127.0.0.1:59982 latency=121.301µs ```

So did some debugging and found that:
### LockFlagValue is getting changed when i update the key value from the UI.
// LockFlagValue is a magic flag we set to indicate a key
// is being used for a lock. It is used to detect a potential
// conflict with a semaphore.

I'm struck here, Since i dont have any other process running to acquire a lock or trying to create a semaphore parallely when i run my program.

Start with lock.go line 207 and trace the LockOpts path, LockFlagValue handling, and the Consul KV update behavior. Reproduce the sequence in a local dev-mode Consul: acquire the lock, edit the key in the UI, and acquire it again while inspecting the stored pair. Done requires a confirmed explanation of the changed lock metadata and a clearly scoped behavior or fix.

Written by the indexing model from the issue text.