When making selection sets, the first 100 features (for ArcMap) or 1000 features (for ArcPro) are cached on the client. Above that threshold, they get stored in the database in SDE_LOGFILES and SDE_LOGFILE_DATA. When accessing read-only databases, ArcMap or ArcPro cannot write in those tables and therefore are limited to that threshold. In previous ArcGIS Desktop versions it was possible to modify that threshold by using a registry key. After some exchange with ESRI support, I was told that this solution wasn’t supported anymore. That’s sad! It would be very helpful in situation like the one described above. Even nicer, would be to bring back the option of altering that threshold but from the user interface instead of a registry key.