Wait, are you tapping while the loaf is still hot out of the oven? That hollow drum sound actually happens because steam is still moving around inside the crust, so a hot loaf can sound hollow even when it needs more time. I let mine cool for at least an hour before I trust any tap test, and the pitch usually drops as it sits. Pulling loaves 4 minutes early based on a hot tap could be why the crumb opened up, but it might just be underbaked rather than a real doneness signal.

My buddy runs a shop up in Wichita and had the exact same fight with an adjuster over a quarter panel on a Silverado, and he ended up caving because the customer was breathing down his neck about the bill. Six months later the seam started showing rust bubbles right at the arch, and guess who got blamed for it. Stand your ground, because that comeback is gonna land on you, not the guy in the office.

Sell the mismatched stuff now and stop paying rent on junk you hate.