When I set the font size over 148+, the text color change to white

I am running ubuntu-mate on my raps-pi 4b,
I found something very interesting when I set the canvas's font size over 140,
the font color will change to white,even you set the color to other color in the beginning.(I am using lazarus)

then i open the office-wirter,set a text's size to 150,the same thing happened.

any one met this before?