DOCBOOK-APPS: wrong sorting in index in output of 1.58.1

[ABX <abx@abx.art.pl>]

I had correctly sorted index in either long xhtml and FO/PDF
output generated with xsltproc -> DocBook XSL 1.58.1 -> ...

Symbols, A, C, D, E, F, G, H, I, L, M, N, O, R, S, T, V, Y

when I added following three indexterms in the middle of document:

<indexterm><primary>frame_step</primary><secondary>Reference, Frame_Step option</secondary></indexterm>
<indexterm><primary>STP</primary><secondary>Reference, Frame_Step option</secondary></indexterm>
<indexterm><primary>+STP</primary><secondary>Reference, Frame_Step option</secondary></indexterm>

then my index reordered itself during rendering to:

Symbols, S, A, C, D, E, F, G, H, I, L, M, N, O, R, T, V, Y

As you see S jumped to begining but after Symbols.

Any idea what could be wrong ? Could it be becouse I used
the same content for <escondary> in those options ?
Could it be becouse of plus sign in third <primary> ?

ABX