I've been trying to put together a schema/test that parses 3 different characters, and then has three sequences (all part of a top-level sequence with '|' as the separator). The first sequence will have the first character as a separator, the second sequence will use the second character, and so on. So this is my input:

.|-|#|1.2.3|1-2-3|3#9#1

debug output:

..... 

(debug) 

separator [1]: xs:string: -

  infoset:

    <ex:e2 xmlns:ex="http://example.com">

      <ex:a1>.</ex:a1>

      <ex:a1>-</ex:a1>

      <ex:a1>#</ex:a1>

      <ex:a2>

        <ex:num>1</ex:num>

      </ex:a2>

    </ex:e2>

  data:

           │ (7 to 7)

    .|-|#|1.2.3|1-2-3|3#9#1

  failure:

    Parse Error: Sep('CompiledExpression({ fn:trace(xs:string(/ex:e2/ex:a1[dfdl:occursIndex()]), 'separator') })') - sequence: Delimiter not found!

    Schema context: sequence Location line 32 column 116 in file:/tmp/daffodil_tmp_7144462590172026171.dfdl.xsd

    Data location was preceding byte 7

    UTF-8 text starting at byte 0 is: (.|-|#|1.2.3|1-2-3|3#9#1)

    Data (hex) starting at byte 0 is: (0x2E7C2D7C237C312E322E337C312D322D337C3323392331)

The output from sbt is similar:

separator [1]: xs:string: .

separator [1]: xs:string: -

[error] Test edu.illinois.ncsa.daffodil.section16.array_optional_elem.TestArrayOptionalElem.test_arrayExpressions02 failed: edu.illinois.ncsa.daffodil.processors.ParseError: Parse Error: Sep('CompiledExpression({ fn:trace(xs:string(/ex:e2/ex:a1[dfdl:occursIndex()]), 'separator') })') - sequence: Delimiter not found!

It seems to correctly find the separator in order to fill in the num element, but still fails.

I created a few more tests to try and narrow down the issue:
arrayExpressions02b - demonstrates that you can use an expression referencing a previous element to set the separator
arrayExpressions02c - similar to arrayExpressions02, except the expression with occursCount is used for the terminator property of the innermost element.
arrayExpressions02d - similar to arrayExpressions02, except the expression with occursCount is used for the initiator of the top-level element instead of the sequence separator.

It seems that this is only a problem when used in a property of the sequence.

see test arrayExpressions02 in the following files:
daffodil-test/src/test/scala-debug/edu/illinois/ncsa/daffodil/section16/array_optional_elem/TestArrayOptionalElemDebug.scala
daffodil-test/src/test/scala/edu/illinois/ncsa/daffodil/section16/array_optional_elem/TestArrayOptionalElem.scala
daffodil-test/src/test/resources/edu/illinois/ncsa/daffodil/section16/array_optional_elem/ArrayOptionalElem.tdml