I submitted a support ticket regarding the same issue with OneDrive search not working, and received the following reply:
We have carefully reviewed your concern regarding the search feature appreciate your feedback and your effort in bringing this issue to our attention. Regrettably, we must inform you that this is a known issue that our Engineering team has acknowledged*. They are currently investigating the matter and are actively working towards a resolution. This issue is a priority in our development pipeline, and we are committed to resolving it promptly.*
We extend our gratitude for your assistance in identifying this issue. We encourage you to continue checking for app updates as we anticipate releasing a fix in the near future. Please note that we do not have a specific ETA (Estimated Time of Arrival) at this time as we are still in the process of developing the solution. In the meantime, we suggest you manually navigate through the OneDrive folders structures..
If you submit any support requests (about OneDrive search not working) you will get a similar response and your ticket will be closed. This is because its a known issue and we will just have to wait for a server-side fix.