Although the bags of mulch fit in the Model Y if you check the Model Y Owner's Manual the maximum weight allowance for the cargo area floor is just 130 lbs. You may have overloaded the cargo area floor.
Load Limits - Secure all cargo before moving Model Y. Place heavy cargo in the lower trunk compartment.
Caution: To avoid damage, never load more than 130 lbs (60 kg) on the rear load floor (above the lower trunk compartment) or more than 285 lbs (130
kg) in the lower trunk compartment. Doing so can cause damage.
I thought the same thing. 2cu ft of mulch, dry, is about 20 pounds, can easily go to 40 pounds if wet. 400-800 lbs?
I think the OP would know by now if he damaged the flat floor of the cargo area, or the hinges of the seat backs.
I will be doing the same thing in a few days, but will probably limit it to 10 bags per trip (HD/Lowes only 1 mile from my house)
One thing for sure. The OP has found a way to dampen the harmonic drumming coming from the rear of some of these cars!