I have a survey that currently contains almost 1900 records in the primary feature class (Shifts), and 17,428 records in the related table (Inspections). Exporting the whole database to Excel from the Survey123 data page results in an output file containing all of the Shifts records, but only 9168 of the Inspections. The missing records are randomly spaced throughout the data, so it's not that the last half of data is being truncated by Excel. Same thing if I export a CSV. I get an error exporting a fGDB, so I can't compare that.
If I sync with ArcGIS Desktop, I do see all of the records in both tables. I also tried exporting an Excel file from the Overview for the survey feature class within ArcGIS Online, and did get the correct number of records. I'm now concerned because I've been exporting data for clients for two months now, not realizing that I may not have been giving them all of their data. I have absolutely no filters or selections on the data when I export it.