[Koha] DOM versus grs-1

Hello, I recently upgraded a site from Koha 3.6 to 3.8.5 (via 3.8.3). Shortly afterwards we noticed that there was quite a few biblios not appearing on the search results even though they exist in the database. After trying many, many things I decided to switch from DOM to grs-1 and rebuild the indexes. This has resolved the problem and the search results are now correct. Switching back to DOM brings back the problem. Does anyone know why this might be the case - broadly speaking. Should DOM and grs create the same index result set by using different filters or are these types of differences expected? Thanks Barry