Hi Berno
When you buy this ticket on SBB you get two tickets: one for ÖBB (Bregenz - St. Margrethen) and one for SBB (St. Margethen - Luzern):

If you buy this ticket on ÖBB you get only one:

On 13 October 2025 the train doesn’t stop at the border station of St. Margrethen due to engineering works and the ÖBB ticketing system is unable to issue the ticket Bregenz - St. Margrethen requested by SBB.
I would buy the ticket on ÖBB, but be aware that ÖBB-tickets are no more refundable if you download them as PDF.
Kind regards
Danhiel