Good afternoon and thank you for your question. I tested /me/onlineMeetings and see the response and the joinWebURL result is by design. Alternatively, you can use URL shortening links but that will be out of my expertise.
I highly suggest posting the feature suggestion on Microsoft Graph Feedback Portal - https://feedbackportal.microsoft.com/feedback/forum/ebe2edae-97d1-ec11-a7b5-0022481f3c80. I hope I was able to assist you with your question. If yes, please Upvote and click on Accept as an answer.
Have a great day ahead!