With Siril 1.03 using the one star registration setting and a small box around the zero point star, I could then sum stack mono spectra of stars taken with the slitless Star Analyzer filter and see the stacked spectrum in the spectroscopy software RSPEC.
With version 1.21 and the same stacking settings, the registering and stacking are reported by Siril to have worked correctly but the stacked fits image just results in two points with no spectra in RSPEC. A fits viewer (Astropixel Processor) shows that the stacked 1.21 spectrum image looks to be intact and I have checked and there appears no issue deriving from the file name itself.
When I wrote this query a few minutes ago I thought Siril 1.21 was somehow changing the properties of the registered or stacked fits files differently to Siril 1.03. But now that I have removed 1.21 and reinstalled 1.03, I am having the same problem so it seems to be some subtlety in the Siril registration or stacking setting although there is not many options there to get wrong.
A bit more progress on this issue, I find if I use a file converter to change the stacked Siril fits file into 16 bit format, it now seems to work in RSPEC (RSPEC is supposed to work with 32 bit files and I did have the RSPEC 32 bit option enabled)
Unless I have just been unlucky with my choice of stars or conditions I am not sure what I can do.
While I am here, this may well have been asked before but since Siril is fussy about numbering, it would also be nice if there was a functionality to convert a lot of camera image files to a numbered sequence or perhaps have an option for Siril be less fussy about differing characters (temperature etc) in the file names prior to the end number.