This is interesting. Can’t say I’ve seen it before, but I’m presuming that UIStroke is a separately rendered UI element to the text rather than being rendered as one solid object. Hence you can see it rendering underneath the transparent text. You can only ‘see’ it when the text is boldened is because when your text is standard, the UIStroke is thick enough to fill the entire inside of the text. (see how the text on the left is darker, the stroke is still visible behind it)
A way to fix this would be to put the TextLabel inside a CanvasGroup and tween the GroupTransparency instead. A CanvasGroup is like a Frame, except it “bakes” all of the UI inside into a cached image whenever a child property changes - and you can control the tint colour and transparency of the finalised image. For memory optimisation you probably don’t want to use toooo many, but generally they’re excellent to make use of