diff --git a/lib/private/Files/Cache/CacheEntry.php b/lib/private/Files/Cache/CacheEntry.php
index 74af7d0a59800..5f20bfdd3e613 100644
--- a/lib/private/Files/Cache/CacheEntry.php
+++ b/lib/private/Files/Cache/CacheEntry.php
@@ -140,7 +140,7 @@ public function __clone() {
 
 	#[\Override]
 	public function getUnencryptedSize(): int {
-		if ($this->data['encrypted'] && isset($this->data['unencrypted_size']) && $this->data['unencrypted_size'] > 0) {
+		if ($this->data['encrypted'] && isset($this->data['unencrypted_size'])) {
 			return $this->data['unencrypted_size'];
 		} else {
 			return $this->data['size'] ?? 0;
diff --git a/lib/private/Files/Cache/Scanner.php b/lib/private/Files/Cache/Scanner.php
index 36c8c02a8bfca..df73ccd79d94d 100644
--- a/lib/private/Files/Cache/Scanner.php
+++ b/lib/private/Files/Cache/Scanner.php
@@ -183,8 +183,12 @@ public function scanFile($file, $reuseExisting = 0, $parentId = -1, $cacheData =
 						}
 					}
 
-					// we only updated unencrypted_size if it's already set
-					if (isset($cacheData['unencrypted_size']) && $cacheData['unencrypted_size'] === 0) {
+					// Only skip updating unencrypted_size if both cached and new values are 0
+					// This allows updating from incorrect cached 0 to correct non-zero value
+					// while avoiding unnecessary updates when both are legitimately 0
+					if (isset($cacheData['unencrypted_size'])
+						&& $cacheData['unencrypted_size'] === 0
+						&& (!isset($data['unencrypted_size']) || $data['unencrypted_size'] === 0)) {
 						unset($data['unencrypted_size']);
 					}
 
@@ -202,7 +206,12 @@ public function scanFile($file, $reuseExisting = 0, $parentId = -1, $cacheData =
 						$data['etag_changed'] = true;
 					}
 				} else {
-					unset($data['unencrypted_size']);
+					// For new files, preserve unencrypted_size only when the file is encrypted
+					// and the key is present; otherwise clear it so it won't be written stale.
+					if (!isset($data['encrypted']) || !$data['encrypted']
+						|| !isset($data['unencrypted_size'])) {
+						unset($data['unencrypted_size']);
+					}
 					$newData = $data;
 					$fileId = -1;
 				}
diff --git a/lib/private/Files/FileInfo.php b/lib/private/Files/FileInfo.php
index 65280b0f31580..37d96ba27ff2b 100644
--- a/lib/private/Files/FileInfo.php
+++ b/lib/private/Files/FileInfo.php
@@ -173,7 +173,7 @@ public function getSize($includeMounts = true) {
 		if ($includeMounts) {
 			$this->updateEntryFromSubMounts();
 
-			if ($this->isEncrypted() && isset($this->data['unencrypted_size']) && $this->data['unencrypted_size'] > 0) {
+			if ($this->isEncrypted() && isset($this->data['unencrypted_size'])) {
 				return $this->data['unencrypted_size'];
 			} else {
 				return isset($this->data['size']) ? 0 + $this->data['size'] : 0;
diff --git a/lib/private/Files/Storage/Wrapper/Encryption.php b/lib/private/Files/Storage/Wrapper/Encryption.php
index 8edf25e9a5111..a256dfaa63d1d 100644
--- a/lib/private/Files/Storage/Wrapper/Encryption.php
+++ b/lib/private/Files/Storage/Wrapper/Encryption.php
@@ -400,6 +400,7 @@ protected function verifyUnencryptedSize(string $path, int $unencryptedSize): in
 		if ($unencryptedSize < 0
 			|| ($size > 0 && $unencryptedSize === $size)
 			|| $unencryptedSize > $size
+			|| ($unencryptedSize === 0 && $size > $this->util->getHeaderSize())
 		) {
 			// check if we already calculate the unencrypted size for the
 			// given path to avoid recursions
diff --git a/lib/private/Files/Stream/Encryption.php b/lib/private/Files/Stream/Encryption.php
index 1c662aa023c6c..7b83ce0592a80 100644
--- a/lib/private/Files/Stream/Encryption.php
+++ b/lib/private/Files/Stream/Encryption.php
@@ -28,7 +28,7 @@ class Encryption extends Wrapper {
 	protected string $cache;
 	protected ?int $size = null;
 	protected int $position;
-	protected ?int $unencryptedSize = null;
+	protected int|float|null $unencryptedSize = null;
 	protected int $headerSize;
 	protected int $unencryptedBlockSize;
 	protected array $header;