After much digging around, I think I may have found the culprit. Since the brightness change is random, I can't say for certain it's resolved, but here are the steps I've taken so far:
Open Windows Power Options and open Lenovo's Power Manager from the sidebar on the left. (You can also locate this application by typing "Power Manager" in Start Search.)
In the Lenovo Power Manager, switch to "Advanced" mode.
Under the active power profile, ensure that the "monitor brightness" setting is set to the desired level.
Under the "Global Power Settings" tab, make sure all "Dynamic Brightness Control" options are disabled.
Under the "Options" tab, make sure "Magic Brightness Control" is disabled.
Press OK and hope for the best.