Do not reuse streaming decoder

lib/fetch/body.js

@@ -27,7 +27,6 @@ let ReadableStream = globalThis.ReadableStream
 /** @type {globalThis['File']} */
 const File = NativeFile ?? UndiciFile
 const textEncoder = new TextEncoder()
-const textDecoderNoBom = new TextDecoder('utf-8', { ignoreBOM: true })
 const textDecoder = new TextDecoder()
 
 // https://fetch.spec.whatwg.org/#concept-bodyinit-extract
@@ -445,13 +444,16 @@ function bodyMixinMethods (instance) {
           let text = ''
           // application/x-www-form-urlencoded parser will keep the BOM.
           // https://url.spec.whatwg.org/#concept-urlencoded-parser
+          // Note that streaming decoder is stateful and cannot be reused
+          const streamingDecoder = new TextDecoder('utf-8', { ignoreBOM: true })
+
           for await (const chunk of consumeBody(this[kState].body)) {
             if (!isUint8Array(chunk)) {
               throw new TypeError('Expected Uint8Array chunk')
             }
-            text += textDecoderNoBom.decode(chunk, { stream: true })
+            text += streamingDecoder.decode(chunk, { stream: true })
           }
-          text += textDecoderNoBom.decode()
+          text += streamingDecoder.decode()
           entries = new URLSearchParams(text)
         } catch (err) {
           // istanbul ignore next: Unclear when new URLSearchParams can fail on a string.