I suggest reporting this as a possible bug in the Report an Issue part of this forum. (see tabs at top of this screen.)
Here's a sample screen capture of what I am seeing. These are both taken from the same database. Note the account numbers in the F1 field. The capture from MS Access displays what is indeed the highest account number in the database and it is correct. The FM Pro screen capture I took shows the highest account number displayed of the SAME file by FM Pro. This is a very serious issue for us and is extremely problematic. The first screen capture March CD is from Filemaker Pro.
This is the same database opened with MS Access which correctly displays the highest account number in the database. Note that FM Pro is displaying 465,741 LESS accounts/entries in this database. Note in the search field I did a search for the highest account number in the database which is 118746116. No records found. The operative question is WHY?
Thank you for your posts and screen shots. The Microsoft Access file shows 9,776,320 records while the FileMaker Pro database only shows 3,970,345 records, so there are approximately 5,800,000 records missing; not 400,000.
From Excel, export the records to a comma-separated values or tab-separated values text file. Then, import the text file into both Access and FileMaker Pro. Does this work? If so, then there is something in the .xlsx file that FileMaker Pro is having difficulty reading. What version of Excel is being used to create the file?