Applying glow ink on signs doesn't render the text bright without also having applied a custom color

When glow ink sacs are used on a sign, they make the text render bright but since all signs have black text this has no effect - the text remains black like always. Applying a dye to the text fixes the problem.

What I expected to happen:

I expected the text on the sign to become readable in the dark when clicking it with a glow ink sac, without also needing to dye the text.

What actually happens:

The text on the sign remains black and unreadable in the dark, not appearing glowing at all, when clicking it with a glow ink sac, unless the sign was previously dyed.

Steps to reproduce:

1. Find a dark spot, preferably lock yourself in a cage of tinted glass

2. Place a sign somewhere in the dark spot, and write some text on it
   ✔  The text is not/barely readable because it's too dark

3. Click the sign with a glow inc sac
   ❌  The text is still not/barely readable because the text was and still is black

4. Click the sign with any dye (except black dye)
   ✔  The text becomes readable because it is no longer black, and it glows properly

Video:

https://youtu.be/T2ZFtXpAD1c