The PLY format starts with the ply magic word (#23018)

examples/jsm/loaders/PLYLoader.js

@@ -88,7 +88,7 @@ class PLYLoader extends Loader {
 
 		function parseHeader( data ) {
 
-			const patternHeader = /ply([\s\S]*)end_header\r?\n/;
+			const patternHeader = /^ply([\s\S]*)end_header\r?\n/;
 			let headerText = '';
 			let headerLength = 0;
 			const result = patternHeader.exec( data );