The Lucee implementation of FileExists produces results which are inconsistent with the ACF implementation when the given 'source' argument is a valid relative path but not a valid absolute path (i.e. the path is only valid if ExpandPath() is used)

Consistent results are obtained if the optional second argument 'allowRealPath' is explicitly passed as FALSE, but code written with this argument won't compile under ACF

https://docs.lucee.org/reference/functions/fileexists.html

For clarity, this argument should be renamed to 'allowRelativePath'. Description in documentation should be updated to something like:
'boolean indicating whether the source argument may be a relative path. If false, the returned value will only be true if the source is a valid absolute path. If true, the returned value will also be true if ExpandPath(source) returns a valid absolute path

For improved functionality, this argument should be replaced with a string argument named 'pathType' which accepts one of three values:
'absoluteOnly' (file path interpreted as absolute, must be default for ACF compatibility)
'relativeOnly' (file path interpreted as relative to a mapping - equivalent to calling FileExists(ExpandPath(source)) in ACF
'relativeAllowed' (source checked as absolute first, then as relative if absolute path isn't found)

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To reproduce in a Linux environment:

1) configure a mapping '/issues/' which resolves to a local directory such as '/home/content/public/issues/'
2) put a file 'foo.txt' into this directory
3) run the following code on Lucee and ACF - observe different results:

WriteOutput(FileExists("/issues/foo.txt"));

// Lucee returns 'true', ACF returns false

WriteOutput(FileExists("/issues/foo.txt",false));

// Lucee returns 'false', ACF returns error 'Parameter validation error for the FILEEXISTS function'