Confirmed, this is indeed strange behaviour, I think a bug.

It happens because Org supports both letter/number references to cells (like A7, or in
your case r1) and also Org's default representation, something like @7$1 (equivalent to
A7) or @1$18 (equivalent to r1). Wherever appropriate, it converts A7-like references to
@7$1-like references. See more in

    (info "(org) References")

, and specifically the "Named references" section. You can evaluate that info sexp, or
find it in section 3.5.1 otherwise :).

It seems like a mistake to convert such a cell reference in the '$PROP_x' case. The
conversion is done by 'org-table-convert-refs-to-rc', which is done before
'org-table-formula-substitute-names' is called. Simply changing the order will not work,
since the property value might also be a letter/number-like reference.

My added patch fixes this by not replacing such a representation when it is preceded by
'$PROP_'.

I have two questions about it:

- The function '$PROP_<remote' still gives '#ERROR', is this related to this bug, or is
  the property name '<remote' invalid in any way and is this intended?

- Should I add tests for this behaviour?

Best,
Rens