According to the information you provide which has helped me lot for my test, and here are my test results, you can see the difference in the screenshots:
On Mac Os Mojave 16.14.6, PowerPoint for Mac 16.16.15, the question you mentioned was completely reproduced in my test environment. Then I tested these versions: 16.16.14, 16.16.12, 16.16.10, and got the same outcome.
But on Mac OS Catalina 10.15, PowerPoint 16.16.15, freeform drawing feature is working.
Based on the above results, the cause of your problem is most likely caused by a Mac OS updating, to solve your problem, I suggest you can choose to upgrade to Catalina. At the same time, you can click on the smiley face icon in the upper
right corner of the PowerPoint to send feedback.