I have the same issue although not to the same extend. The number of orphan omap mapping is much less and it is only in the Data container. I tried to use DiskUtility in the Recovery mode but the result is similar. Every so often another orphan omap mapping appears. I have no idea whether it is affecting the performance bc I don't see any difference (yet?)
Could anyone advise me what to do?
BTW I am running Big Sur 11.7.3 on MBP