The Anime Man has some interviews with people working in the anime industry. In the one with a creator of Redo of Healer and the one with a Ǝ voice actress, the interviewees wear masks, while in the one with a normal innocent anime voice actress, the interviewee does not wear any mask. It seems that the 2 interviewees working in the hentai industry do not want others to know that.
Besides, there's also a similar phenomenon in Kakushigoto, where the father doesn't want her daughter to know that he's an eromanga sensei. Here's the synopsis from MyAnimeList.
Kakushi Gotou is a somewhat popular manga artist whose works are known for inappropriate content. Because of this raunchiness, when his daughter Hime was born, he vowed to keep his profession hidden from her, believing that she will be disillusioned if she finds out.
This paranoia-induced belief leads Kakushi into hectic situations. Despite being a single father, he does his best and often resorts to extreme ends just to protect his secret, such as guising as a salaryman every day or holding emergency drills in case Hime somehow finds her way to his workplace.
These materials make me wonder whether a job in the hentai industry is widely frowned upon in Japan.