Weird air pockets/bubbles on bottom of cake
That happened to me as well. Luckily it doesn't change the taste, but it is annoying! It comes from air pockets which formed because you whisked/mixed your cake batter too intensely, and/or for too long, which creates air bubbles. So first, it is best to mix your ingredients delicately, try to avoid incorporating too much air (easier said than done), and stop whisking once your batter reached the desired homogeneous consistency (overwhisking is a common mistake). Then, when you pour your cake batter in the pan, it's better to avoid pouring it all at once in the center, it will also create air bubbles. It's better to pour it delicately from one corner and let it spread by itself in the dish. Finally, before putting in the oven, tap (delicately) the bottom of your pan on the counter to free the remaining air bubbles and let them reach the surface before cooking.
I hope that will help, let me know!