Turnstile block blanks out when replying to comments on smartphones

When users reply to an existing comment using the standard, out-of-the-box WordPress Comments system, the box appears either all black or all white, depending on the theme setting of the plugin. I don’t see it when using my Windows laptop, but I do see it on my and my wife’s Google Pixel smartphones. Doesn’t matter if we’re logged in, cookieless, cache-cleared– it appears that the common denominator is smartphone (and not simulated smartphone).