Open Batch files's order is not processed alphabetically as expected

We have set a single open batch multi-period for which all the required files may be found under one directory.

We have the correct file id for each file, which mostly is formed with the date-username-entityname-priority@Location@Category@PeriodStart@PeriodEnd@RM.txt, but the open batch does not process the files in the expected alphabetical order. It looks like it kinda processes the files in group of same category first, instead of following exatcly the file id pattern. Example : all Forecast files first, and all Budget files after... Based on which logic ?

Does the issue may come from the fact that we have multiple files tagged for more than one (in this case 2) categories /data load rules, under the same directory ? We use the same location to load Forecast and Budget data files with the same open batch.