add explanation about format mismatch

platform-includes/distributed-tracing/custom-instrumentation/native.mdx

@@ -41,4 +41,4 @@ To create a transaction as a continuation of a trace retrieved from an upstream
 
 ```
 
-The format of the `sentry-trace` header should follow the one defined in [the telemetry docs](https://develop.sentry.dev/sdk/telemetry/traces/#header-sentry-trace). It should consist of a 32 character hexadecimal string for the `traceId`, followed by a 16 character hexadecimal string for the `spanId` and an optional single character for the `sampled` flag.
+The format of the `sentry-trace` header should follow the one defined in [the telemetry docs](https://develop.sentry.dev/sdk/telemetry/traces/#header-sentry-trace). It should consist of a 32 character hexadecimal string for the `traceId`, followed by a 16 character hexadecimal string for the `spanId` and an optional single character for the `sampled` flag. If the given string doesn't match this format, the update is ignored and the values in the transaction context will remain unchanged.