You can achieve the same thing without even the need of \n or <br>.
Just write your content as it is, if you need it to be multi-lined. For example:
"abc
abc
abc"
All you have to do is to add this to your global css:
::ng-deep .mat-tooltip {
white-space: pre-line !important;
}
UPDATE 2021
The above solution is pretty old and ::ng-deep is deprecated as pointed out by @bsplosion in the comments.
If you don't want to use the solution of @rodris you can alternatively just edit the class directly. Add this to your global stylesheet:
.mat-tooltip {
white-space: pre-line;
}
` or `