titleformat frame not recognized on framebreak numbers

[ghost]

If I set titleformat frame=allsmallcaps and use a frame with allowframebreaks enabled only the title will be set in smallcaps but not the corresponding latin number (e.g. I,II,III). This looks very strange in my opinion.

The following code will show the problem if titleformat frame=allsmallcaps is set:
\begin{frame}[allowframebreaks]{Title} foo \newpage foo \newpage foo \newpage foo \end{frame}

Here is a hint how to solve the problem.

[matze]

This looks very strange in my opinion.

Indeed it does but that's what numerals (no matter if roman or arab) look like when typeset in small caps with Fira. This is an issue with a long history (see #33 for first report) that lead to the current implementation allowing you to choose different frametitle styles. Besides that, I don't think we can do much about it.

[ghost]

I'm not sure if you're right, atleast the following code shows that these numerals will have its own smallcaps also with Fira.

\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage[T1]{fontenc}
\usepackage[sfdefault]{FiraSans}
%\usepackage[default]{sourcesanspro}
\begin{document}
Xx \romannumeral 3 \textsc{\romannumeral 3} III
\end{document}

My assumption was that the scshape command is not passed to beamer's \insertcontinuationcount.
I think it could be easily changed by override this command to use scshape if smallcaps are requested in options.

Edit:
In fact this was the problem, here is a solution (only the if check for smallcaps needed):

\setbeamertemplate{frametitle continuation}{\textsc{\romannumeral\insertcontinuationcount}}

[matze]

Sorry about the confusion. You were right!

[ghost]

No problem, thanks for having a look at it. Unfortunately we face new problems with the new solution.
With allsmallcaps all is fine now, with smallcaps we get smallcaps for the latin numerals. I think it's okay to make the latin numerals all smallcaps or allcaps in this case (maybe from designers view the allcaps are prefered here, from the logical view the allsmallcaps would be chosen).
But the cases allcaps and no option will fail for the latin numerals, it seems they are just lowercase now.
I think the easiest way would be to add an if-check as mentioned above and only change frametitle continuation if allsmallcaps is given and maybe the thing I wrote about smallcaps.

(Atleast that's what I think when I copied your change to my preambel)