Hello and welcome to the forum bminer.
I've never experienced what you've described, and I use plenty of models with multiple external lookups to xlsx files.
I'd be tempted to test with csv files instead of xlsx. Open the files in Excel and save as MS-DOS CSV, then rebuild your lookups to use the csv files.
If you're still seeing odd data in the fields, you might want to verify that those values are really in those fields incorrectly. Garbage In Garbage Out and all that. :confused:
As I say, this is unusual behavior. While rare, it's possible that the model is corrupt. Rebuilding a model from scratch when you know what you want to structure really doesn't take too long at all usually, so you might want to try that too.