What version are you on? Wondering if the calculation tool is/was buggy in previous versions. I thought I saw a post about that somewhere but can’t find it now. Still digging…
Found it. This individual was seeing the same thing. The Calculation widget was concatenating a string instead of actually performing an addition function.
Does look like he circled back with any resolution (he was using 2024.1.11)…
At this point I would report this as a bug and see if they have any kind of fix/work around for your current version.