Is this a grayscale bug or working as intended?

[jasonmnemonic]

Summary
Is this a grayscale bug or working as intended?

I have a grayscaled image in which I have added 2 coloured text in yellow. This file is loaded to TUI. I clicked to grayscale and everything changes to grayscale. Next, I undo grayscale and picture is still grayscaled but text are now in yellow. This is expected.

Next, I added a third yellow text. This time, I hit grayscale. Now, everything turns to grayscale except the third yellow text. When I unclicked grayscale, the third yellow text is also grayscaled. It is basically stuck at yellow.

If I save this image with all three text as yellow and I reload the image from file. Now clicking grayscale will make everything (all three text) become grayscale.

Is this a bug or feature?

Version
I use TUI version 3.0.2 but you can also see this on 18th July 2023 on the http:https://nhn.github.io/tui.image-editor/latest/tutorial-example01-includeUi website.