Welcome to the forum. My guess is the temperature of the butter. Most recipes call for room temperature butter when creaming which is incorrect. Beating causes friction heat. Too much heat and your butter will break. Below is a link to a post on the proper way to cream butter. It was written by Stella Parks, a James Beard award winning cookbook author. I have been creaming butter in this method for about 20 years; so I can attest to the accuracy of Parks method.
Sarah Phillips who has a website called Craftybaking also talks about creaming butter using cold butter. Link below.
Using butter at 65°F ( 18°C) is taught in most culinary baking programs. I don’t understand why recipes for the home baker instructs use of room temperature butter. Room temperature butter is just simply too warm to cream.
Spreadable butter is blended with oil to keep it pliable. That may also be contributing to the problem. When I need my butter to be more pliable, such as when I am laminating dough, I use Kerrygold butter. How butter is tempered in the manufacturing process will determine how pliable, hard, or brittle it is when chilled. Kerrygold has perfected the art of tempering butter to create a nice pliable butter that stays pliable when chilled.
Correct way to cream butter
https://www.seriouseats.com/2015/12/cookie-science-creaming-butter-sugar.html
Scroll down to the section title “Butter Temperature”
https://www.craftybaking.com/howto/mixing-method-creaming