This issue is not reproducible when testing with current Survey123 for ArcGIS web app production release 3.1. Using the provided xlsx example and constraint, the web app is working as expected. The survey is unable to submit unless 5 digits are entered into the Employee ID field. If 4 or 6 digits are entered the question is highlighted in red and display the custom constraint message. If try and submit with 4 or 6 digits then an error message is displayed at top of web form, this is expected. If 5 digits are entered, the survey is submitted successfully. If the Employee ID field is left blank, the survey is also submitted as the question is not required.
Note that the constraint message set for Employee ID is not accurate, it states that 'Employee ID must be less than 5 number' but the constraint used is forcing the answer to only be 5 digits 'regex(., '^\d{5}$')'. The warning message should be update to say that the 'Employee ID must be 5 number'. More information on regular expressions can be found here: https://doc.arcgis.com/en/survey123/desktop/create-surveys/xlsformformulas.htm#ESRI_SECTION1_8979CBF8B7354DA9A4FE0F6B6D61E1A0
Note also that the 'BIND Employee ID <= 5 chars' used in bind::esri:warning column is also not a valid expression.