update: It is also possible that you did not need the spacer/cap in the first space. You state you have a Rotor BB. Didn't you try to use a cap that allows using thinner cranks? For example allows an GPX crank with a Hollowtech II BB. In that case the cap would just be something in the way when using a Hollowtech II crank and so no wonder it would be forcibly and destructively pushed out. The link in the text below show another possibility. A cap that allows Hollowtech II cranks in a wider BB.
Maybe you just must be more careful when installing the crank. I cannot think of any special trick that would have to be done to avoid damaging the caps. If there is any significant resistance, stop and try to fit the crank better. At the and one may need a few hits by a mallet, but that us usually when it already sticks out, just not fully.
I originally thought it was part of the sleeve present in Shimano bottom brackets because the question said "Hollowtech II".
These parts of a Hollowtech II bottom bracket are integral parts of the BB. They connect the opposite bearing cups and separate the bearing from the spindle. You fix them when you screw the left side bearing cup. Then when inserting the crank there should not be any potential to damage it if it was installed correctly. The spindle should go inside reasonably smoothly.
One option for an incorrect install might be using the incorrect spacer in between. E.g. using a 73 mm spacer for a 68 mm bb shell and then screwing the left cup (the force it generates is big). It would then bulge and partially block the way for the spindle. The final damage would be done when forcefully inserting the crank.
Make sure using the correct spacers for your bottom bracket shell on your frame as shown in Difference between Road/Mountain Hollowtech II Bottom Brackets?
There are some spare pieces sold, but not widely available. For example https://www.chainreactioncycles.com/shimano-hollowtech-ii-central-sleeve/rp-prod7428 (this piece is the big one between the cups) and https://www.kineticbikebearings.com/kbbb9037.html (thanks to Swifty for the name). There is usually no need for them so they are not so common in the e-shops. The links are only illustrational. I do not know about your local bike shop, but it is worth asking. You may need a whole new BB.