Quick answer: Accessibility bugs are invisible to most testers. A bash with accessibility-focused testers (or simulated impairments) surfaces them.
Accessibility is a feature; the bugs are the parts that broke. Test them deliberately.
Simulate impairments
Color-blindness filters; small-text reading; controller-only navigation. Each surfaces specific bugs.
Hire accessibility consultants
External testers with lived experience. Catch things simulations miss.
Test against WCAG-ish
Adapted for games. Color contrast, text size, controller mapping. Each criterion is a test case.
Tag accessibility bugs
Separate tag in the tracker. Prioritize together; ensure not lost in general triage.
“Accessibility bashing is a specialized skill. Build the bench.”
If you don't have accessibility on the team, contract for it. Quarterly review of access bugs is the minimum.