On Android S, memory size not displayed accurately

It might be useful to install the AIDA64 app, look under System and see whether the figures are repeated.

I do not know the full history of the phone and reinstall history [1]; I suppose it might be worth ruling out any unusual failure in the install by doing a full reinstall. https://doc.e.foundation/devices/FP3/install

I do not remember a similar fault mentioned I think it would be taken seriously in a stable build if you Report an issue – ideally, imho, before rooting.

[1] Is known: Feedback for v1.11 - #139 by Januario