Stairs don't count as full-blocks when using the filter 'is_underground'

For my addon, I use the filter 'is_underground', which then triggers an event that gives me a tag. And it will remove that tag if I'm not underground. However, when I was testing it out more, I realized that stairs don't count as being underground with the filter. This has been a bug for a while, however, I can't pinpoint the exact time it started for me. But I was hoping you could look into this because this will really help out with the addon that I'm creating.

 

Unfortunately, my video was too big to send through the attachments. However, I have a YouTube video link instead: https://www.youtube.com/watch?v=9x-XGRs-OYU