First, create a class for your tooltip, to make it break lines on line breaks.
.my-tooltip {
white-space: pre-line;
}
Then, add the class to your tooltip and insert line breaks with the Unicode code
<span
matTooltip="First line Second line"
matTooltipClass="my-tooltip">
</span>
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…