Also consider lower resolution tiles as used tiles

src/ol/TileCache.js

@@ -21,8 +21,7 @@ class TileCache extends LRUCache {
   expireCache(usedTiles) {
     while (this.canExpireCache()) {
       const tile = this.peekLast();
-      const zKey = tile.tileCoord[0].toString();
-      if (zKey in usedTiles && usedTiles[zKey].contains(tile.tileCoord)) {
+      if (tile.getKey() in usedTiles) {
         break;
       } else {
         this.pop().dispose();