A quick guess is that the browser does not have access to the particular fonts you are using in the slides. You can probably look up a list of fonts that work well for online display. I personally switched to beautiful AI, not because I wanted slides made by AI, but because Slide decks are quicker to build and easy to embed in Canvas. I will often throw the slides into Canvas with an embed and then present straight from Canvas by making the slides full screen.
You might also get good results from using the web version of PowerPoint to build the slide decks, and then using one of the Microsoft integrations into Canvas to share the decks. Although my understanding is that Microsoft is rolling out a new Canvas integration on September 15th.