Requirements for restoring a Time Machine backup from a sparsebundle?
I have experienced problems using sparsebundles on APFS volumes, whereby writing ~50gb lead to a backup that had incorrect checksums (determined by running tmutil verifychecksums and seeing a failure).

I then switch to using sparsebundles on HFS+ volumes, and no more problems.

Worth mentioning that you are probably using sparsebundles that are themselves HFS+ formatted, if you are using them for Time Machine.

If I had to take a guess, I would say it probably has more to do with the filesystem that the sparsebundle lives on (not the filesystem internal to the sparsebundle)... and less to do with the read/write vs read-only issue.  You can test this simply...  try to mount your sparsebundle while it lives on a read-only HFS+ volume... does that work? (can you mount?)

Yes, I realize that the filesystem where the sparsebundle lives should not matter, but I have myself seen that sometimes it does.  One reason is quite simply that filesystems themselves have bugs.  APFS, NTFS, and HFS are no exception to this.

As some further detail, I noticed that verifying checksums for a sparsebundle on an APFS volume took significantly longer than performing the same operation on a similar sparsebundle on an HFS+ volume.