The problem will re-occur if you continue storing the Mini with the rear blades crossed.Changed the blades twice and stills happened.
I missed if you supplied the recipe for the CORRECT way to fold the rear props in to avoid distortion.
Here are four more images to illustrate the problem with the rear propeller blades.
The rear motors are mounted on an angle, canted outwards. Here you can see how the rear propeller blades point upward when turned out.
When the prop blades are turned inward (across the body of the Mini) you can see how fresh new (straight) blades extend away from the body of the Mini. If the blades from both sides are turned inwards they form an X with the tips crossing. The blades from one side will not fit under the blades from the other side unless one side or the other is bent to fold under the opposite blades. If your rear blades now slide easily together it can only mean they're already bent.
Obviously if you set it down on a tabletop with the blades positioned like this, the weight of the Mini will bend the blades. The Fly More case has a trough into which the rear blades can hang without the weight of the Mini resting on them. However, just forcing the blades into the required crossed position is already enough to bend the blades. So even if stored "properly" in the Fly More case, the rear blades are bending.
If you instead position the blades longitudinally down the sides of the Mini, the angle the motors are mounted at is level with the rear arms. You can set it down flat on a tabletop like this without bending the prop blades. However, they will not fit into the Fly More case, nor into the original plastic shipping tray of non-fly-more Minis, with the prop blades positioned like this.
I have been storing my Mini on a shelf with the arms open unfolded ever since replacing the propeller blades. For travelling, I ordered a new carry case which stores the prop blades positioned longitudinally, both front and rear blades down the sides of the Mini rather than laterally folded across the body.
Search for a "Sunnylife Mavic Mini Case". There are different types. This one allows the blades to be stored positioned longitudinally.