skillsaves filtered?

[Lombax99]

i'm trying to do a tool in obsidian to generate statblock dinamically using fantasy statblock and templater.
I've created a bunch of dynamic strings like:
<%+ tp.frontmatter.saves[0] + ": +" + tp.user.getBonus(tp.frontmatter.stats, tp.frontmatter.cr, tp.frontmatter.saves[0]) %>
or
<%+ tp.user.getBonus(tp.frontmatter.stats, tp.frontmatter.cr, tp.frontmatter.saves) %>
but for the skillsaves sometimes they do not work.
After some test i believe the statblock, before rendering, makes some check like having ": +" somewhere. Is that correct?
I've noticed this because of the two example above only the first one work even though both return the same exact string once rendered. (It doesn't look like they would but i've changed the getBonus function accordingly and have made many, many test to check)