I would like to work out why you have had an issue. It does seem you are having permission issues writing the file but NFC why it's not happened before.
Let me get back to you when I have the code in front of me and some spare time.

As for checking Swap Used on eMMC is was removed for two reasons, one it didn't calculate it correctly so the values are actually incorrect and finally it messed up calculating the uSD swap correctly.
FlopSwap only supports swapping to uSD (in the set locations in the wiki) so is pointless if you only have swap on internal.
The whole idea is internal swap is only used as a backup for memory card removal.


Edit:-

@Il_Principe
Can you check to see if /etc/sudoers.d/flopswap.sudoers has been created? If it is can you PM me the contents.
Do you use bb-power or bash?
Also could you check/send me a copy of the permissions table for the files in question.