Custom fonts will look different when uploaded to your talk. When using an unsupported font, we will attempt to substitute the font for the closest matching font available. Embedding fonts in your presentation can help resolve the issue: https://support.microsoft.com/en-gb/office/embed-fonts-in-documents-or-presentations-cb3982aa-ea76-4323-b008-86670f222dbc
Click here to learn more about Slides feature on Talks.